diff --git a/gufe/tests/test_pdbstructure.py b/gufe/tests/test_pdbstructure.py
new file mode 100644
index 00000000..7e8f3a1b
--- /dev/null
+++ b/gufe/tests/test_pdbstructure.py
@@ -0,0 +1,46 @@
+from io import StringIO
+
+from gufe.vendor.pdb_file.pdbstructure import PdbStructure, _parse_atom_index
+
+
+def test_hex_conect_parsing():
+    pdb_snippet = """ATOM  99999  C   LIG A   1       0.000  0.000  0.000  1.00 0.00           C
+ATOM  A000F  N   LIG A   2       1.000  0.000  0.000  1.00 0.00           N
+ATOM  A000G  O   LIG A   3       0.000  1.000  0.000  1.00 0.00           O
+CONECT99999A000FA000G"""
+
+    f = StringIO(pdb_snippet)
+    pdb = PdbStructure(f, load_all_models=True)
+
+    # Collect all atom serial numbers, including Maestro-style
+    atom_serials = [atom.serial_number for atom in pdb.iter_atoms(use_all_models=True)]
+
+    # There should be 3 atoms
+    assert len(atom_serials) == 3
+
+    # All serial numbers should be integers and unique
+    for serial in atom_serials:
+        assert isinstance(serial, int)
+    assert len(set(atom_serials)) == 3
+
+    # Convert the known Maestro-style indices to integers
+    a000f_serial = _parse_atom_index("A000F")
+    a000g_serial = _parse_atom_index("A000G")
+    assert a000f_serial in atom_serials
+    assert a000g_serial in atom_serials
+
+    # Check that CONECT records refer to the correct integers
+    conects = pdb._current_model.connects
+    assert len(conects) == 1
+    central, bonded1, bonded2 = conects[0]
+    assert central == 99999
+    # The bonded atoms match the converted Maestro-style serials
+    assert bonded1 in [a000f_serial, a000g_serial]
+    assert bonded2 in [a000f_serial, a000g_serial]
+    assert bonded1 != bonded2
+
+    known_hex_serials = ["A000G", "A000F"]
+    for hex_serial in known_hex_serials:
+        idx = _parse_atom_index(hex_serial)
+        assert isinstance(idx, int)
+        assert idx >= 100000
diff --git a/gufe/vendor/pdb_file/pdbstructure.py b/gufe/vendor/pdb_file/pdbstructure.py
index cda68915..6a7b6af0 100644
--- a/gufe/vendor/pdb_file/pdbstructure.py
+++ b/gufe/vendor/pdb_file/pdbstructure.py
@@ -165,13 +165,16 @@ def _load(self, input_stream):
             if command == "ATOM  " or command == "HETATM":
                 self._add_atom(Atom(pdb_line, self, self.extraParticleIdentifier))
             elif command == "CONECT":
-                atoms = [_parse_atom_index(pdb_line[6:11])]
-                for pos in (11, 16, 21, 26):
-                    try:
-                        atoms.append(_parse_atom_index(pdb_line[pos : pos + 5]))
-                    except:
-                        pass
-                self._current_model.connects.append(atoms)
+                try:
+                    atoms = [_parse_atom_index(pdb_line[6:11])]
+                    for pos in (11, 16, 21, 26):
+                        try:
+                            atoms.append(_parse_atom_index(pdb_line[pos : pos + 5]))
+                        except:
+                            pass
+                    self._current_model.connects.append(atoms)
+                except:
+                    pass
             # Notice MODEL punctuation, for the next level of detail
             # in the structure->model->chain->residue->atom->position hierarchy
             elif pdb_line[:5] == "MODEL":
@@ -1060,12 +1063,27 @@ def __str__(self):
             return str(self.position)
 
 
-def _parse_atom_index(index):
-    """Parse the string containing an atom index, which might be either decimal or hex."""
-    try:
-        return int(index)
-    except:
-        return int(index, 16) - 0xA0000 + 100000
+def _parse_atom_index(index: str) -> int:
+    """
+    Parse an atom serial index from a PDB file, supporting:
+      - Decimal numbers (e.g., 12345)
+      - Standard hex (0–9, A–F)
+      - Maestro-style extended hex (letters beyond F, e.g., A000G)
+    """
+    index = index.strip()
+
+    # Try decimal, then hex, then Maestro base36
+    for base in (10, 16, 36):
+        try:
+            val = int(index, base)
+            if val >= 0xA0000:
+                val = val - 0xA0000 + 100000
+            return val
+        except ValueError:
+            continue
+
+    raise ValueError(f"Unable to parse atom index: '{index}'")
+
 
 
 # run module directly for testing