diff --git a/src/ifc5d/ifc5d/ifc5Dspreadsheet.py b/src/ifc5d/ifc5d/ifc5Dspreadsheet.py
index 5aab765c26..a0f12e2535 100644
--- a/src/ifc5d/ifc5d/ifc5Dspreadsheet.py
+++ b/src/ifc5d/ifc5d/ifc5Dspreadsheet.py
@@ -21,6 +21,7 @@ from __future__ import annotations
 
 import argparse
 import datetime
+import json
 import logging
 import os
 import time
@@ -256,26 +257,28 @@ class IfcDataGetter:
             return ""
         if cost_item.CostQuantities is None:
             return ""
-        string = "["
+        result = []
         for quantity in cost_item.CostQuantities:
-            string += '["'
+            prefix = ""
             for rel in file.get_inverse(quantity):
                 if rel.is_a("IfcPropertySet") or rel.is_a("IfcElementQuantity"):
-                    prop_set = rel
                     # Find elements that have this property set
-                    for prop_rel in file.get_inverse(prop_set):
+                    for prop_rel in file.get_inverse(rel):
                         if prop_rel.is_a("IfcRelDefinesByProperties"):
                             for obj in prop_rel.RelatedObjects:
                                 if obj.is_a("IfcElement"):
-                                    string += obj.Name + " - "
-            string += quantity.Name
+                                    prefix += (obj.Name or "") + " - "
+            name = prefix + (quantity.Name or "")
             if quantity.is_a("IfcPhysicalSimpleQuantity"):
-                string += '", ' + str(quantity[3]) + "],"
+                value = quantity[3]
+                try:
+                    value = float(value) if value is not None else 0.0
+                except (TypeError, ValueError):
+                    value = 0.0
+                result.append([name, value])
             else:
-                string += ' ERROR: Only IfcPhysicalSimpleQuantity is supported", 0.0],'
-        string = string.removesuffix(",")
-        string += "]"
-        return string
+                result.append([name + " ERROR: Only IfcPhysicalSimpleQuantity is supported", 0.0])
+        return json.dumps(result, ensure_ascii=False)
 
 
 class SheetData(TypedDict):
diff --git a/src/ifc5d/test/test_csv2ifc.py b/src/ifc5d/test/test_csv2ifc.py
index c14a795da3..a630bb44ac 100644
--- a/src/ifc5d/test/test_csv2ifc.py
+++ b/src/ifc5d/test/test_csv2ifc.py
@@ -17,6 +17,7 @@
 # along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
 
 import csv
+import json
 import tempfile
 from pathlib import Path
 
@@ -118,3 +119,25 @@ class TestCsv2Ifc:
             writer.write()
             assert len(list(Path(temp_csv_dir).glob("*.ods"))) == 1
             assert len(list(Path(temp_csv_dir).glob("*.xlsx"))) == 1
+
+
+class TestSerialiseCostQuantities:
+    def test_quantity_name_with_special_characters_round_trips_as_json(self):
+        ifc_file = ifcopenshell.file()
+        name = 'Prospetto est "Np=256,667-23"'
+        quantity = ifc_file.create_entity("IfcQuantityArea", Name=name, AreaValue=12.5)
+        cost_item = ifc_file.create_entity("IfcCostItem", CostQuantities=[quantity])
+
+        result = ifc5d.ifc5Dspreadsheet.IfcDataGetter.serialise_cost_quantities(ifc_file, cost_item)
+
+        assert json.loads(result) == [[name, 12.5]]
+
+    def test_unset_name_does_not_crash(self):
+        ifc_file = ifcopenshell.file()
+        # Name left unset so quantity.Name resolves to None at access time.
+        quantity = ifc_file.create_entity("IfcQuantityArea", AreaValue=3.0)
+        cost_item = ifc_file.create_entity("IfcCostItem", CostQuantities=[quantity])
+
+        result = ifc5d.ifc5Dspreadsheet.IfcDataGetter.serialise_cost_quantities(ifc_file, cost_item)
+
+        assert json.loads(result) == [["", 3.0]]