diff --git a/Cargo.toml b/Cargo.toml
index 56ea6ca..d06402d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,6 +26,8 @@ strum = { workspace = true, features = ["derive"] }
 
 [dev-dependencies]
 pyo3 = { workspace = true }
+rstest = { workspace = true }
+assert-json-diff = { workspace = true }
 
 [features]
 pyo3 = ["dep:pyo3", "dep:pythonize"]
@@ -37,7 +39,9 @@ path = "tests/lib.rs"
 [workspace.dependencies]
 pyo3 = "0.21.1"
 pythonize = "0.21.1"
+rstest = "0.21.0"
 serde = "1.0"
 serde_json = "1.0"
 strum = "0.26.2"
 uuid = "1.4"
+assert-json-diff = "2.0.2"
diff --git a/src/lib.rs b/src/lib.rs
index f529722..5410829 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -10,19 +10,3 @@ pub mod pytket;
 
 pub use circuit_json::SerialCircuit;
 pub use optype::OpType;
-
-#[cfg(test)]
-mod tests {
-    use crate::circuit_json::SerialCircuit;
-    use serde_json::to_string;
-    #[test]
-    fn read_json() {
-        let circ_s = r#"{"bits": [["c", [0]], ["c", [1]]], "commands": [{"args": [["q", [0]]], "op": {"type": "H"}}, {"args": [["q", [0]], ["q", [1]]], "op": {"type": "CX"}}, {"args": [["q", [0]], ["c", [0]]], "op": {"type": "Measure"}}, {"args": [["q", [1]], ["c", [1]]], "op": {"type": "Measure"}}], "implicit_permutation": [[["q", [0]], ["q", [0]]], [["q", [1]], ["q", [1]]]], "phase": "0.0", "qubits": [["q", [0]], ["q", [1]]]}"#;
-        let ser: SerialCircuit = serde_json::from_str(circ_s).unwrap();
-
-        assert_eq!(ser.commands.len(), 4);
-
-        let reser: SerialCircuit = serde_json::from_str(&to_string(&ser).unwrap()[..]).unwrap();
-        assert_eq!(ser, reser);
-    }
-}
diff --git a/src/opbox.rs b/src/opbox.rs
index 10862a6..ae8f30e 100644
--- a/src/opbox.rs
+++ b/src/opbox.rs
@@ -117,7 +117,7 @@ pub enum OpBox {
         /// The phase polynomial definition.
         /// Represented by a map from bitstring to expression of coefficient.
         phase_polynomial: Vec<Vec<(Bitstring, String)>>,
-        ///
+        /// The additional linear transformation.
         linear_transformation: Matrix,
     },
     /// A user-defined assertion specified by a list of Pauli stabilisers.
diff --git a/tests/data/diagonal-box.json b/tests/data/diagonal-box.json
new file mode 100644
index 0000000..b6e1c2b
--- /dev/null
+++ b/tests/data/diagonal-box.json
@@ -0,0 +1,137 @@
+{
+    "bits": [],
+    "commands": [
+        {
+            "args": [
+                [
+                    "q",
+                    [
+                        0
+                    ]
+                ],
+                [
+                    "q",
+                    [
+                        1
+                    ]
+                ],
+                [
+                    "q",
+                    [
+                        2
+                    ]
+                ]
+            ],
+            "op": {
+                "box": {
+                    "control_state": 1,
+                    "id": "0dadbfea-5391-41c2-ad71-fb01fdaecddd",
+                    "n_controls": 1,
+                    "op": {
+                        "box": {
+                            "diagonal": [
+                                [
+                                    [
+                                        1.0,
+                                        0.0
+                                    ]
+                                ],
+                                [
+                                    [
+                                        1.0,
+                                        0.0
+                                    ]
+                                ],
+                                [
+                                    [
+                                        0.7071067811865476,
+                                        0.7071067811865475
+                                    ]
+                                ],
+                                [
+                                    [
+                                        0.9238795325112867,
+                                        0.3826834323650898
+                                    ]
+                                ]
+                            ],
+                            "id": "919b6c13-b9c4-4ffc-ad1a-660088402d02",
+                            "type": "DiagonalBox",
+                            "upper_triangle": true
+                        },
+                        "type": "DiagonalBox"
+                    },
+                    "type": "QControlBox"
+                },
+                "type": "QControlBox"
+            }
+        }
+    ],
+    "created_qubits": [],
+    "discarded_qubits": [],
+    "implicit_permutation": [
+        [
+            [
+                "q",
+                [
+                    0
+                ]
+            ],
+            [
+                "q",
+                [
+                    0
+                ]
+            ]
+        ],
+        [
+            [
+                "q",
+                [
+                    1
+                ]
+            ],
+            [
+                "q",
+                [
+                    1
+                ]
+            ]
+        ],
+        [
+            [
+                "q",
+                [
+                    2
+                ]
+            ],
+            [
+                "q",
+                [
+                    2
+                ]
+            ]
+        ]
+    ],
+    "phase": "0.0",
+    "qubits": [
+        [
+            "q",
+            [
+                0
+            ]
+        ],
+        [
+            "q",
+            [
+                1
+            ]
+        ],
+        [
+            "q",
+            [
+                2
+            ]
+        ]
+    ]
+}
\ No newline at end of file
diff --git a/tests/data/simple.json b/tests/data/simple.json
new file mode 100644
index 0000000..855dbaa
--- /dev/null
+++ b/tests/data/simple.json
@@ -0,0 +1,133 @@
+{
+    "bits": [
+        [
+            "c",
+            [
+                0
+            ]
+        ],
+        [
+            "c",
+            [
+                1
+            ]
+        ]
+    ],
+    "commands": [
+        {
+            "args": [
+                [
+                    "q",
+                    [
+                        0
+                    ]
+                ]
+            ],
+            "op": {
+                "type": "H"
+            }
+        },
+        {
+            "args": [
+                [
+                    "q",
+                    [
+                        0
+                    ]
+                ],
+                [
+                    "q",
+                    [
+                        1
+                    ]
+                ]
+            ],
+            "op": {
+                "type": "CX"
+            }
+        },
+        {
+            "args": [
+                [
+                    "q",
+                    [
+                        0
+                    ]
+                ],
+                [
+                    "c",
+                    [
+                        0
+                    ]
+                ]
+            ],
+            "op": {
+                "type": "Measure"
+            }
+        },
+        {
+            "args": [
+                [
+                    "q",
+                    [
+                        1
+                    ]
+                ],
+                [
+                    "c",
+                    [
+                        1
+                    ]
+                ]
+            ],
+            "op": {
+                "type": "Measure"
+            }
+        }
+    ],
+    "implicit_permutation": [
+        [
+            [
+                "q",
+                [
+                    0
+                ]
+            ],
+            [
+                "q",
+                [
+                    0
+                ]
+            ]
+        ],
+        [
+            [
+                "q",
+                [
+                    1
+                ]
+            ],
+            [
+                "q",
+                [
+                    1
+                ]
+            ]
+        ]
+    ],
+    "phase": "0.0",
+    "qubits": [
+        [
+            "q",
+            [
+                0
+            ]
+        ],
+        [
+            "q",
+            [
+                1
+            ]
+        ]
+    ]
+}
\ No newline at end of file
diff --git a/tests/roundtrip.rs b/tests/roundtrip.rs
new file mode 100644
index 0000000..d7d85cf
--- /dev/null
+++ b/tests/roundtrip.rs
@@ -0,0 +1,26 @@
+//! Roundtrip tests
+use assert_json_diff::assert_json_include;
+use rstest::rstest;
+use serde_json::Value;
+use tket_json_rs::SerialCircuit;
+
+const SIMPLE: &str = include_str!("data/simple.json");
+const DIAGONAL: &str = include_str!("data/diagonal-box.json");
+
+#[rstest]
+#[case::simple(SIMPLE, 4)]
+#[case::diagonal_box(DIAGONAL, 1)]
+fn roundtrip(#[case] json: &str, #[case] num_commands: usize) {
+    let initial_json: Value = serde_json::from_str(json).unwrap();
+    let ser: SerialCircuit = serde_json::from_value(initial_json.clone()).unwrap();
+
+    assert_eq!(ser.commands.len(), num_commands);
+
+    let reencoded_json = serde_json::to_value(&ser).unwrap();
+    // Do a partial comparison. The re-encoded circuit does not include "created_qubits" nor "discarded_qubits".
+    assert_json_include!(actual: initial_json, expected: reencoded_json);
+
+    let reser: SerialCircuit = serde_json::from_value(reencoded_json).unwrap();
+
+    assert_eq!(ser, reser);
+}