diff --git a/Sources/Schema/RelaxNGParser.swift b/Sources/Schema/RelaxNGParser.swift
index 4c35bd7..7fe4278 100644
--- a/Sources/Schema/RelaxNGParser.swift
+++ b/Sources/Schema/RelaxNGParser.swift
@@ -52,16 +52,31 @@ private final class RNGCompiler {
             mergeInclude(include)
         }
         for define in RNGNode.children(node, named: "define") {
-            if let name = RNGNode.attribute(define, "name") {
-                defines[name] = combined(RNGNode.elementChildren(define), .sequence)
-            }
+            addDefine(define)
         }
         let start = RNGNode.children(node, named: "start").first
         return start.map { combined(RNGNode.elementChildren($0), .sequence) } ?? .notAllowed
     }
 
+    /// Adds a `define`, honoring `combine`: a second definition of the same name
+    /// with `combine="choice"`/`"interleave"` merges with the existing one rather
+    /// than replacing it (a plain redefinition still replaces).
+    private func addDefine(_ define: Tree) {
+        guard let name = RNGNode.attribute(define, "name") else { return }
+        let pattern = combined(RNGNode.elementChildren(define), .sequence)
+        guard let existing = defines[name] else {
+            defines[name] = pattern
+            return
+        }
+        switch RNGNode.attribute(define, "combine") {
+        case "choice": defines[name] = .choice(existing, pattern)
+        case "interleave": defines[name] = .interleave(existing, pattern)
+        default: defines[name] = pattern
+        }
+    }
+
     /// Merges an `include`d grammar: its `define`s first, then the `include`'s own
-    /// nested `define`s, which override the included ones.
+    /// nested `define`s, which override or (with `combine`) merge with them.
     private func mergeInclude(_ node: Tree) {
         guard let href = RNGNode.attribute(node, "href"), !visited.contains(href),
               let text = loader(href), let root = try? PureXML.parseTree(text),
@@ -71,14 +86,10 @@ private final class RNGCompiler {
         }
         visited.insert(href)
         for define in RNGNode.children(grammar, named: "define") {
-            if let name = RNGNode.attribute(define, "name") {
-                defines[name] = combined(RNGNode.elementChildren(define), .sequence)
-            }
+            addDefine(define)
         }
         for define in RNGNode.children(node, named: "define") {
-            if let name = RNGNode.attribute(define, "name") {
-                defines[name] = combined(RNGNode.elementChildren(define), .sequence)
-            }
+            addDefine(define)
         }
     }
 
diff --git a/Tests/RelaxNGCombineTests.swift b/Tests/RelaxNGCombineTests.swift
new file mode 100644
index 0000000..2ffd121
--- /dev/null
+++ b/Tests/RelaxNGCombineTests.swift
@@ -0,0 +1,55 @@
+@testable import PureXML
+import Testing
+
+@Suite("RELAX NG combine on define")
+struct RelaxNGCombineTests {
+    private let rngNamespace = "xmlns=\"http://relaxng.org/ns/structure/1.0\""
+
+    private func valid(_ rng: String, _ xml: String) throws -> Bool {
+        try PureXML.Schema.RelaxNG(rng).validate(xml)
+    }
+
+    @Test("combine=choice merges two definitions of the same name as a choice")
+    func test_combineChoice() throws {
+        let rng = """
+        <grammar \(rngNamespace)>
+          <start><ref name="content"/></start>
+          <define name="content"><element name="a"><empty/></element></define>
+          <define name="content" combine="choice"><element name="b"><empty/></element></define>
+        </grammar>
+        """
+        // Both alternatives validate against the merged choice.
+        #expect(try valid(rng, "<a/>"))
+        #expect(try valid(rng, "<b/>"))
+        #expect(try !valid(rng, "<c/>"))
+    }
+
+    @Test("combine=interleave merges two definitions as an interleave")
+    func test_combineInterleave() throws {
+        let rng = """
+        <grammar \(rngNamespace)>
+          <start><element name="root"><ref name="content"/></element></start>
+          <define name="content"><element name="a"><empty/></element></define>
+          <define name="content" combine="interleave"><element name="b"><empty/></element></define>
+        </grammar>
+        """
+        // Interleave admits the two children in either order.
+        #expect(try valid(rng, "<root><a/><b/></root>"))
+        #expect(try valid(rng, "<root><b/><a/></root>"))
+        // Both are required by the interleave, so one alone fails.
+        #expect(try !valid(rng, "<root><a/></root>"))
+    }
+
+    @Test("A plain redefinition without combine still replaces")
+    func test_plainRedefinitionReplaces() throws {
+        let rng = """
+        <grammar \(rngNamespace)>
+          <start><ref name="content"/></start>
+          <define name="content"><element name="a"><empty/></element></define>
+          <define name="content"><element name="b"><empty/></element></define>
+        </grammar>
+        """
+        #expect(try !valid(rng, "<a/>"))
+        #expect(try valid(rng, "<b/>"))
+    }
+}