diff --git a/CHANGELOG.md b/CHANGELOG.md
index a0ecc0c..a4810dd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- XSTS invalid-schemas-accepted (#145): an identity-constraint `selector`/`field` xpath may not use an undeclared namespace prefix. Every prefix in a name test must be a declared `xmlns:` prefix in scope (the implicit `xml` prefix excepted); a `selector`/`field` whose xpath names an unbound prefix is rejected. invalid-schemas-accepted 77 → 75, no false positive (valid-schemas-rejected held at 0; an adversarial review confirmed declared prefixes on the schema root or any ancestor resolve correctly), no other bucket moved.
+
 - XSTS invalid-schemas-accepted (#161): a `group` inside `xs:redefine` may reference itself at most once, and that self-reference must occur exactly once, `minOccurs` = `maxOccurs` = 1 (`src-redefine.6.1.2`). A self-reference with `minOccurs="0"`, `maxOccurs="unbounded"`, or any count above 1 is rejected. The self-reference is matched by namespace, not local name alone, so a reference to an imported group that merely shares the local name is unconstrained (no false positive). invalid-schemas-accepted 79 → 77, no false positive (valid-schemas-rejected held at 0), no other bucket moved.
 
 - XSTS invalid-schemas-accepted (#145): a local `element` declaration must identify itself with a `name` or a `ref`; an element with neither is rejected. invalid-schemas-accepted 80 → 79, no false positive (valid-schemas-rejected held at 0), no other bucket moved.
diff --git a/Sources/Schema/SchemaIdentityXPath.swift b/Sources/Schema/SchemaIdentityXPath.swift
index 740eb62..ab751e8 100644
--- a/Sources/Schema/SchemaIdentityXPath.swift
+++ b/Sources/Schema/SchemaIdentityXPath.swift
@@ -21,9 +21,30 @@ extension PureXML.Schema.XSDParser {
         guard local == "selector" || local == "field",
               let xpath = PureXML.Schema.XSDNode.attribute(node, "xpath")
         else { return [] }
-        return validXPath(xpath, isField: local == "field")
-            ? []
-            : ["the \(local) xpath '\(xpath)' is not a valid identity-constraint path"]
+        if !validXPath(xpath, isField: local == "field") {
+            return ["the \(local) xpath '\(xpath)' is not a valid identity-constraint path"]
+        }
+        // Every namespace prefix a name test uses must be declared in scope; the
+        // implicit `xml` prefix is always bound. An unbound prefix is invalid.
+        let bindings = namespaceBindingsInScope(of: node, defaultBindings: [:])
+        for prefix in xpathNameTestPrefixes(xpath) where prefix != "xml" && bindings[prefix] == nil {
+            return ["the \(local) xpath '\(xpath)' uses the undeclared namespace prefix '\(prefix)'"]
+        }
+        return []
+    }
+
+    /// The set of namespace prefixes used in the name tests of a (syntactically
+    /// valid) identity-constraint xpath. A name test is `prefix:local` or
+    /// `prefix:*`; an unprefixed name, `*`, `.`, or an axis keyword has no prefix.
+    private static func xpathNameTestPrefixes(_ xpath: String) -> [String] {
+        guard let tokens = tokenize(xpath) else { return [] }
+        var prefixes: [String] = []
+        for case let .name(name) in tokens {
+            guard let colon = name.firstIndex(of: ":") else { continue }
+            let prefix = String(name[name.startIndex ..< colon])
+            if !prefix.isEmpty { prefixes.append(prefix) }
+        }
+        return prefixes
     }
 
     private enum XPathToken: Equatable {
diff --git a/Tests/SchemaIdentityXPathTests.swift b/Tests/SchemaIdentityXPathTests.swift
index fc0b16c..9b5dbfa 100644
--- a/Tests/SchemaIdentityXPathTests.swift
+++ b/Tests/SchemaIdentityXPathTests.swift
@@ -13,7 +13,7 @@ struct SchemaIdentityXPathTests {
     private func compile(_ selector: String, fields: [String] = ["@v"]) throws {
         let fieldElements = fields.map { #"<xs:field xpath="\#($0)"/>"# }.joined()
         _ = try PureXML.Schema.Document(#"""
-        <xs:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xs="http://www.w3.org/2001/XMLSchema">
+        <xs:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:ns="urn:ns">
           <xs:element name="root" type="xs:string"/>
           <xs:element name="r">
             <xs:complexType><xs:sequence><xs:element ref="root"/></xs:sequence></xs:complexType>
@@ -85,6 +85,17 @@ struct SchemaIdentityXPathTests {
         return (try? PureXML.Schema.Document(document)) != nil
     }
 
+    /// A name-test prefix in a selector/field xpath must be a declared namespace
+    /// prefix; an undeclared prefix is invalid. The implicitly-bound `xml` prefix and
+    /// a prefix bound by the schema's own `xmlns:` declarations are valid.
+    @Test("an identity-constraint xpath may not use an undeclared namespace prefix")
+    func test_xpathUndeclaredPrefix() throws {
+        try compile("xs:a")
+        try compile("root", fields: ["xs:v"])
+        #expect(rejectsSelector("imp:a"))
+        #expect(rejectsField("imp:v"))
+    }
+
     @Test("an identity constraint requires one selector then one or more fields")
     func test_identityConstraintContentModel() {
         let selector = #"<xs:selector xpath="c"/>"#
diff --git a/Tests/XSTSSuiteTests.swift b/Tests/XSTSSuiteTests.swift
index 42c11d7..7a77652 100644
--- a/Tests/XSTSSuiteTests.swift
+++ b/Tests/XSTSSuiteTests.swift
@@ -149,10 +149,11 @@ struct XSTSSuiteTests {
     /// schemaLocation must be a schema (src-resolve), import namespace matches the
     /// imported target (src-import.3), redefined type derives from itself
     /// (src-redefine.5), local complexType is anonymous, a local element has a name
-    /// or ref, and a redefined group's self-reference occurs exactly once
-    /// (src-redefine.6.1.2).
+    /// or ref, a redefined group's self-reference occurs exactly once
+    /// (src-redefine.6.1.2), and an identity-constraint xpath uses no undeclared
+    /// namespace prefix.
     private let knownSchemaValidRejected = 0
-    private let knownSchemaInvalidAccepted = 77
+    private let knownSchemaInvalidAccepted = 75
     private let knownInstanceValidRejected = 0
     private let knownInstanceInvalidAccepted = 133