diff --git a/tests/test_tro_utils.py b/tests/test_tro_utils.py
index fea30da..3f50c5d 100644
--- a/tests/test_tro_utils.py
+++ b/tests/test_tro_utils.py
@@ -620,6 +620,40 @@ def test_add_performance_mixed_strings_and_refs(
         assert contributed["trov:arrangement"]["@id"] == "arrangement/2"
         assert contributed["trov:boundTo"] == "/mnt/output"
 
+    def test_add_performance_extra_attrs(
+        self, temp_workspace, tmp_path, gpg_setup, trs_profile
+    ):
+        """Test that extra attributes passed to add_performance are included in the performance."""
+        tro = create_tro_with_gpg(
+            filepath=str(tmp_path / "test_tro.jsonld"),
+            gpg_setup=gpg_setup,
+            profile=trs_profile,
+        )
+        (temp_workspace / "input.txt").write_text("x")
+        tro.add_arrangement(str(temp_workspace), comment="A")
+        (temp_workspace / "output.txt").write_text("y")
+        tro.add_arrangement(str(temp_workspace), comment="B")
+
+        tro.add_performance(
+            start_time=datetime.datetime(2024, 1, 1, 10, 0, 0),
+            end_time=datetime.datetime(2024, 1, 1, 11, 0, 0),
+            comment="with attrs",
+            accessed_arrangement="arrangement/0",
+            modified_arrangement="arrangement/1",
+            extra_attributes={"trov:customAttr": "customValue"},
+        )
+        perf = tro.data["@graph"][0]["trov:hasPerformance"][0]
+        assert perf["trov:customAttr"] == "customValue"
+
+        tro.save()
+        tro = create_tro_with_gpg(
+            filepath=str(tmp_path / "test_tro.jsonld"),
+            gpg_setup=gpg_setup,
+            profile=trs_profile,
+        )
+        perf = tro.data["@graph"][0]["trov:hasPerformance"][0]
+        assert perf["trov:customAttr"] == "customValue"
+
 
 class TestTROSigning:
     """Test TRO signing and verification."""
diff --git a/tro_utils/models/performance.py b/tro_utils/models/performance.py
index 6603138..938ef1f 100644
--- a/tro_utils/models/performance.py
+++ b/tro_utils/models/performance.py
@@ -92,6 +92,7 @@ class TrustedResearchPerformance(TROVModel):
     accessed_arrangements: list[ArrangementBinding] = field(default_factory=list)
     contributed_to_arrangements: list[ArrangementBinding] = field(default_factory=list)
     attributes: list[PerformanceAttribute] = field(default_factory=list)
+    extra_attributes: dict[str, Any] = field(default_factory=dict)
 
     # ------------------------------------------------------------------
     # JSON-LD serialisation
@@ -127,6 +128,8 @@ def to_jsonld(self) -> dict[str, Any]:
             result["trov:contributedToArrangement"] = [
                 ref.to_jsonld() for ref in self.contributed_to_arrangements
             ]
+        if self.extra_attributes:
+            result.update(self.extra_attributes)
         return result
 
     @classmethod
@@ -157,6 +160,19 @@ def _parse_refs(value: Any) -> list[ArrangementBinding]:
                 return [ArrangementBinding.from_jsonld(item) for item in value]
             return [ArrangementBinding.from_jsonld(value)]
 
+        known_keys = {
+            "@id",
+            "@type",
+            "rdfs:comment",
+            "trov:wasConductedBy",
+            "trov:hasPerformanceAttribute",
+            "trov:startedAtTime",
+            "trov:endedAtTime",
+            "trov:accessedArrangement",
+            "trov:contributedToArrangement",
+        }
+        extra = {k: v for k, v in data.items() if k not in known_keys}
+
         return cls(
             performance_id=data["@id"],
             comment=data.get("rdfs:comment", ""),
@@ -168,4 +184,5 @@ def _parse_refs(value: Any) -> list[ArrangementBinding]:
                 data.get("trov:contributedToArrangement")
             ),
             attributes=attributes,
+            extra_attributes=extra,
         )
diff --git a/tro_utils/models/tro.py b/tro_utils/models/tro.py
index ebf94d6..19eeb7b 100644
--- a/tro_utils/models/tro.py
+++ b/tro_utils/models/tro.py
@@ -280,10 +280,8 @@ def _normalise_refs(value: Any) -> list[tuple[str, str | None]]:
             attributes=performance_attributes,
         )
 
-        # Merge extra_attributes as raw fields — stored as a side-channel dict
-        # via the JSON-LD round-trip (no typed model needed for forward compat)
         if extra_attributes:
-            trp._extra_attributes = extra_attributes  # type: ignore[attr-defined]
+            trp.extra_attributes = extra_attributes
 
         self.performances.append(trp)
         return trp