diff --git a/CHANGELOG.md b/CHANGELOG.md
index 49043c8..9f3e225 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- XSLT `key()` over a node-set second argument (#82). `key(name, node-set)` (including `key('k', .)`) now unions the matches for every node's string value and de-duplicates, rather than using only the first node's string value.
 - XSLT `method="html"` output (#82). An `xsl:output method="html"` result is now serialized by the HTML rules (void elements like `<br>` emitted without a self-closing slash, raw-text elements like `<script>`/`<style>` left unescaped) instead of as XML.
 - XSLT `xsl:sort case-order` (#82). When `case-order="upper-first"`/`"lower-first"` is set, the sort comparison is case-insensitive with the case used only to break ties among otherwise-equal strings; without it the default codepoint order is kept.
 - XSLT functions `generate-id`, `system-property`, `element-available`,
diff --git a/Sources/XSLT/XSLTFunctions.swift b/Sources/XSLT/XSLTFunctions.swift
index 09b2f5f..b2892b5 100644
--- a/Sources/XSLT/XSLTFunctions.swift
+++ b/Sources/XSLT/XSLTFunctions.swift
@@ -114,8 +114,21 @@ extension PureXML.XSLT {
                 }
                 .adding("key") { arguments, _ in
                     let name = arguments.first?.string ?? ""
-                    let value = arguments.count > 1 ? arguments[1].string : ""
-                    return .nodeSet((keys[name]?[value] ?? []).map { .tree($0) })
+                    guard arguments.count > 1 else { return .nodeSet([]) }
+                    // A node-set second argument unions the matches for each node's
+                    // string value; any other value is used directly as a string.
+                    let values: [String] = if let nodes = arguments[1].nodes {
+                        nodes.compactMap(\.treeNode).map(\.stringValue)
+                    } else {
+                        [arguments[1].string]
+                    }
+                    var matched: [PureXML.Model.TreeNode] = []
+                    for value in values {
+                        for node in keys[name]?[value] ?? [] where !matched.contains(where: { $0 === node }) {
+                            matched.append(node)
+                        }
+                    }
+                    return .nodeSet(matched.map { .tree($0) })
                 }
                 .adding("document") { arguments, _ in
                     guard let uri = arguments.first?.string, let text = loader(uri),
diff --git a/Tests/XSLTKeyNodeSetTests.swift b/Tests/XSLTKeyNodeSetTests.swift
new file mode 100644
index 0000000..f0efebf
--- /dev/null
+++ b/Tests/XSLTKeyNodeSetTests.swift
@@ -0,0 +1,56 @@
+@testable import PureXML
+import Testing
+
+@Suite("XSLT key() with the context node and node-set values")
+struct XSLTKeyNodeSetTests {
+    private let xsl = "xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\""
+
+    private func transform(_ stylesheet: String, _ source: String) throws -> String {
+        try PureXML.XSLT.transform(stylesheet: stylesheet, source: source)
+    }
+
+    private let source = """
+    <root>
+      <item cat="a">1</item><item cat="a">2</item><item cat="b">3</item>
+      <lookup>a</lookup><lookup>b</lookup>
+    </root>
+    """
+
+    @Test("key('k', .) uses the context node's string value")
+    func test_contextNode() throws {
+        let style = """
+        <xsl:stylesheet version="1.0" \(xsl)>
+          <xsl:key name="byCat" match="item" use="@cat"/>
+          <xsl:output method="text"/>
+          <xsl:template match="/"><xsl:for-each select="//lookup"><xsl:value-of select="count(key('byCat', .))"/><xsl:text>,</xsl:text></xsl:for-each></xsl:template>
+        </xsl:stylesheet>
+        """
+        // <lookup>a</lookup> -> 2 items, <lookup>b</lookup> -> 1 item.
+        #expect(try transform(style, source) == "2,1,")
+    }
+
+    @Test("key('k', node-set) unions the matches for every node's value")
+    func test_nodeSetValue() throws {
+        let style = """
+        <xsl:stylesheet version="1.0" \(xsl)>
+          <xsl:key name="byCat" match="item" use="@cat"/>
+          <xsl:output method="text"/>
+          <xsl:template match="/"><xsl:value-of select="count(key('byCat', //lookup))"/></xsl:template>
+        </xsl:stylesheet>
+        """
+        // a -> 2, b -> 1, unioned and de-duplicated -> 3.
+        #expect(try transform(style, source) == "3")
+    }
+
+    @Test("key with a string value still works")
+    func test_stringValue() throws {
+        let style = """
+        <xsl:stylesheet version="1.0" \(xsl)>
+          <xsl:key name="byCat" match="item" use="@cat"/>
+          <xsl:output method="text"/>
+          <xsl:template match="/"><xsl:value-of select="count(key('byCat', 'a'))"/></xsl:template>
+        </xsl:stylesheet>
+        """
+        #expect(try transform(style, source) == "2")
+    }
+}