diff --git a/tests/missing_dspacing_angle.xdi b/tests/missing_dspacing_angle.xdi
new file mode 100644
index 0000000..6ae4c98
--- /dev/null
+++ b/tests/missing_dspacing_angle.xdi
@@ -0,0 +1,22 @@
+# XDI/1.0 EDC/5.02
+# Beamline.name: X11A
+# Column.1: angle deg
+# Column.2: mutrans
+# Element.edge: K
+# Element.symbol: Cu
+# Facility.energy: 2.584 GeV
+# Facility.name: NSLS
+# Facility.xray_source: bend magnet
+# Mono.name: Si(111)
+# Sample.name: Cu
+# Sample.prep: Cu metal foil
+# Sample.temperature: 10 K
+# Scan.edge_energy: 8980.0 eV
+# Scan.start_time: 1992-09-15T01:52:53
+# ///
+# Cu foil, 10K — angle-abscissa fixture (missing Mono.d_spacing on purpose)
+# ---
+#     angle           mutrans
+   .1234500E+02    .1013661E+01
+   .1234600E+02    .9949864E+00
+   .1234700E+02    .9900679E+00
diff --git a/tests/test_MissingFields.py b/tests/test_MissingFields.py
index 0ea160f..d01aa2c 100644
--- a/tests/test_MissingFields.py
+++ b/tests/test_MissingFields.py
@@ -18,5 +18,9 @@ def test_missing_element(self):
         self.assertIn("element", self.errors)
         self.assertEqual(2, len(self.errors["element"]))
 
-    def test_missing_mono(self):
-        self.assertIn("mono", self.errors)
\ No newline at end of file
+    def test_mono_dspacing_not_required_for_energy_abscissa(self):
+        # missing_fields.xdi has Column.1: energy — per XDI/1.0 spec,
+        # Mono.d_spacing is only required when the abscissa is
+        # monochromator angle or encoder step count. So no mono error
+        # should fire despite Mono.d_spacing being absent.
+        self.assertNotIn("mono", self.errors)
\ No newline at end of file
diff --git a/tests/test_MonoDspacingConditional.py b/tests/test_MonoDspacingConditional.py
new file mode 100644
index 0000000..92d9c2a
--- /dev/null
+++ b/tests/test_MonoDspacingConditional.py
@@ -0,0 +1,49 @@
+"""XDI/1.0 spec: Mono.d_spacing is required only when the abscissa
+(Column.1) is monochromator angle or encoder step count. For energy
+or wavelength abscissae, Mono.d_spacing is optional.
+
+Ref: https://github.com/XraySpectroscopy/XAS-Data-Interchange/blob/master/specification/spec.md
+"""
+import os
+import unittest
+from xdi_validator import validate
+
+
+HERE = os.path.dirname(__file__)
+
+
+class TestMonoDspacingConditional(unittest.TestCase):
+
+    def _validate(self, fname):
+        with open(os.path.join(HERE, fname), "r") as f:
+            errors, _data = validate(f)
+        return errors
+
+    def test_angle_abscissa_missing_dspacing_errors(self):
+        errors = self._validate("missing_dspacing_angle.xdi")
+        self.assertIn("mono", errors,
+            "Column.1: angle → Mono.d_spacing must be required")
+
+    def test_angle_abscissa_with_dspacing_ok(self):
+        errors = self._validate("valid_angle_dspacing.xdi")
+        self.assertNotIn("mono", errors,
+            "Column.1: angle with Mono.d_spacing present → no mono error")
+
+    def test_energy_abscissa_missing_dspacing_ok(self):
+        # valid.xdi has Column.1: energy and Mono.d_spacing present —
+        # remove the d_spacing header at read-time (in-memory) and
+        # confirm no mono error fires for an energy abscissa.
+        import io
+        with open(os.path.join(HERE, "valid.xdi"), "r") as f:
+            src = f.read()
+        stripped = "\n".join(
+            line for line in src.splitlines()
+            if not line.lstrip("# ").startswith("Mono.d_spacing")
+        )
+        errors, _ = validate(io.StringIO(stripped))
+        self.assertNotIn("mono", errors,
+            "Column.1: energy without Mono.d_spacing → no mono error")
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/tests/test_WriteToFile.py b/tests/test_WriteToFile.py
index a7d6564..e9cf367 100644
--- a/tests/test_WriteToFile.py
+++ b/tests/test_WriteToFile.py
@@ -14,6 +14,9 @@ def setUp(self):
         self.generate_xdi_result = write_xdi(self.data, os.path.dirname(__file__) + "/generated_valid.xdi" )
         self.invalid_data = copy.deepcopy(self.data)
         self.invalid_data["element"].pop("symbol")
+        # Switch abscissa to angle so d_spacing becomes required per
+        # XDI/1.0 spec; then remove it to exercise the mono error path.
+        self.invalid_data["column"]["1"] = "angle deg"
         self.invalid_data["mono"].pop("d_spacing")
 
 
diff --git a/tests/test_WriteToFileTwo.py b/tests/test_WriteToFileTwo.py
index 626758b..261b6e1 100644
--- a/tests/test_WriteToFileTwo.py
+++ b/tests/test_WriteToFileTwo.py
@@ -14,6 +14,9 @@ def setUp(self):
         self.generate_xdi_result = write_xdi(self.data, os.path.dirname(__file__) + "/generated_valid.xdi" )
         self.invalid_data = copy.deepcopy(self.data)
         self.invalid_data["element"].pop("symbol")
+        # Switch abscissa to angle so d_spacing becomes required per
+        # XDI/1.0 spec; then remove it to exercise the mono error path.
+        self.invalid_data["column"]["1"] = "angle deg"
         self.invalid_data["mono"].pop("d_spacing")
 
 
diff --git a/tests/valid_angle_dspacing.xdi b/tests/valid_angle_dspacing.xdi
new file mode 100644
index 0000000..60e3d25
--- /dev/null
+++ b/tests/valid_angle_dspacing.xdi
@@ -0,0 +1,23 @@
+# XDI/1.0 EDC/5.02
+# Beamline.name: X11A
+# Column.1: angle deg
+# Column.2: mutrans
+# Element.edge: K
+# Element.symbol: Cu
+# Facility.energy: 2.584 GeV
+# Facility.name: NSLS
+# Facility.xray_source: bend magnet
+# Mono.d_spacing: 3.135301
+# Mono.name: Si(111)
+# Sample.name: Cu
+# Sample.prep: Cu metal foil
+# Sample.temperature: 10 K
+# Scan.edge_energy: 8980.0 eV
+# Scan.start_time: 1992-09-15T01:52:53
+# ///
+# Cu foil, 10K — angle abscissa with Mono.d_spacing present
+# ---
+#     angle           mutrans
+   .1234500E+02    .1013661E+01
+   .1234600E+02    .9949864E+00
+   .1234700E+02    .9900679E+00
diff --git a/xdi_validator/XDI_validator.py b/xdi_validator/XDI_validator.py
index 4d80700..845b857 100644
--- a/xdi_validator/XDI_validator.py
+++ b/xdi_validator/XDI_validator.py
@@ -408,7 +408,6 @@ def get_schema() -> dict:
                     "name": {"type": "string"},
                     "d_spacing": {"type": "number"},
                 },
-                "required": ["d_spacing"],
             },
             "detector": {
                 "description": "Tags related to the details of the photon detection system",
@@ -566,6 +565,37 @@ def get_schema() -> dict:
             },
         },
         "required": ["version", "subversion", "element", "mono"],
+        "allOf": [
+            {
+                "$comment": (
+                    "XDI/1.0 spec: 'The Mono.d_spacing header field must be "
+                    "specified if the abscissa is conveyed as monochromator "
+                    "angle.' d_spacing is required only when Column.1 "
+                    "(the abscissa) is angle or encoder; for energy or "
+                    "wavelength abscissae, d_spacing is optional. "
+                    "See https://github.com/XraySpectroscopy/"
+                    "XAS-Data-Interchange/blob/master/specification/spec.md"
+                ),
+                "if": {
+                    "properties": {
+                        "column": {
+                            "properties": {
+                                "1": {
+                                    "pattern": "^(angle|encoder)(\\s|$)"
+                                }
+                            },
+                            "required": ["1"]
+                        }
+                    },
+                    "required": ["column"]
+                },
+                "then": {
+                    "properties": {
+                        "mono": {"required": ["d_spacing"]}
+                    }
+                }
+            }
+        ],
     }
 
     return schemadef