diff --git a/CHANGELOG.md b/CHANGELOG.md
index 527a87d..edb2a12 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -58,6 +58,7 @@ QueryPath Changelog
 - Consolidate the sibling and ancestor walks behind `next()`, `prev()`, `nextAll()`, `prevAll()`, `nextUntil()`, `prevUntil()`, `parent()`, `parents()`, `parentsUntil()` and `closest()` into one helper, so a fix to one no longer has to be repeated in its mirror (issue #115). As a side effect, `parentsUntil()` no longer loops forever on an element that is not attached to the document, and `nextUntil('')` and `prevUntil('')` treat an empty selector as no selector, as the other methods already did, instead of throwing `QueryPath\CSS\ParseException`. `closest('')` returns an empty set, as in jQuery. `nextUntil()`, `prevUntil()` and `parentsUntil()` also stop at an element already collected from an earlier starting element, as `nextAll()` and `prevAll()` do, so `nextUntil()` on each of 2000 siblings falls from about 100 ms to 1 ms
 - **Behaviour change.** `prevAll()`, `prevUntil()`, `nextAll()`, `nextUntil()`, `siblings()`, `parent()` and `closest()` now return the results for a multi-element set in jQuery's order, instead of grouped by starting element (issue #117). `prevAll()` and `prevUntil()` return reverse document order, as `parents()` and `parentsUntil()` already did, and the others return document order. `qp($xml, 'c')->add('d')->prevAll()` now returns `c, b, a` rather than `b, a, c`. `next()` and `prev()` keep the order of the starting elements, as in jQuery, and the results for a single starting element are unchanged
 - **Behaviour change.** A positional pseudo-class (`:first`, `:eq()`, `:last`, ...) in the stop selector of `nextUntil()`, `prevUntil()` and `parentsUntil()` now counts across the whole document, as jQuery's does, instead of seeing each sibling or ancestor as a one-element set (issue #118). `nextUntil(':first')` used to stop at once and now walks to the end, and `nextUntil(':eq(3)')` stops at the fourth element in the document. `closest()` no longer matches a positional selector at all, as in jQuery, where `closest(':first')` used to return the element itself. `next()`, `prev()` and `parent()` still test each candidate on its own, which agrees with jQuery for a single starting element
+- **Behaviour change.** `is()` now counts a positional pseudo-class (`:first`, `:last`, `:eq()`, `:lt()`, `:gt()`, `:odd`, `:even`) across the whole document, as jQuery's `.is()` does, instead of within the match set (issue #140). It returns `true` if any element of the set is among the document's matches, so `qp($xml, '#c')->is(':first')` is now `false`. `filter()` still counts within the set. `is()` with a `DOMNode` or `SplObjectStorage` argument, or with a selector that has no positional pseudo-class, is unchanged
 
 # 4.2.0
 
diff --git a/src/Helpers/NodeMatcher.php b/src/Helpers/NodeMatcher.php
index 8df96fd..31fe818 100644
--- a/src/Helpers/NodeMatcher.php
+++ b/src/Helpers/NodeMatcher.php
@@ -107,6 +107,31 @@ public function matchesInDocument($node): bool
 		return $this->inDocument === false ? $this->matches($node) : $this->inDocument->offsetExists($node);
 	}
 
+	/**
+	 * Test whether at least one of the given nodes matches as jQuery's .is() tests it.
+	 *
+	 * @see matchesInDocument()
+	 *
+	 * @param SplObjectStorage $nodes
+	 *
+	 * @return bool
+	 * @throws ParseException
+	 */
+	public function matchesAnyInDocument(SplObjectStorage $nodes): bool
+	{
+		if (! Util::selectorHasPositionalPseudoClass($this->selector)) {
+			return $this->matchesAny($nodes);
+		}
+
+		foreach ($nodes as $node) {
+			if ($this->matchesInDocument($node)) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
 	/**
 	 * @param \DOMDocument $document
 	 *
diff --git a/src/Helpers/QueryChecks.php b/src/Helpers/QueryChecks.php
index 8073a94..723b04f 100644
--- a/src/Helpers/QueryChecks.php
+++ b/src/Helpers/QueryChecks.php
@@ -31,6 +31,10 @@ trait QueryChecks
 	 *
 	 * Unlike jQuery's version, this supports full selectors (not just simple ones).
 	 *
+	 * As in jQuery, a positional selector (`:first`, `:eq()`, ...) counts across the whole
+	 * document, not within the match set: `is(':first')` is TRUE only if the set holds the
+	 * document's first element. filter() counts within the set.
+	 *
 	 * Non-element nodes in the match set can never match a CSS selector, and are skipped.
 	 *
 	 * @param string|DOMNode $selector
@@ -73,7 +77,7 @@ public function is($selector): bool
 			throw new Exception('Cannot compare an object to a DOMQuery.');
 		}
 
-		return (new NodeMatcher($selector))->matchesAny($this->matches);
+		return (new NodeMatcher($selector))->matchesAnyInDocument($this->matches);
 	}
 
 	/**
diff --git a/tests/Issues/Issue140Test.php b/tests/Issues/Issue140Test.php
new file mode 100644
index 0000000..494ae9b
--- /dev/null
+++ b/tests/Issues/Issue140Test.php
@@ -0,0 +1,83 @@
+<?php
+/**
+ * @file
+ *
+ * Regression tests for https://github.com/GravityPDF/querypath/issues/140
+ *
+ * is() counts a positional pseudo-class across the document, as jQuery's .is()
+ * does, where filter() counts it within the set. The expected values are
+ * jQuery 3.7.1's.
+ */
+
+namespace QueryPathTests;
+
+use SplObjectStorage;
+
+class Issue140Test extends TestCase
+{
+	// Document order: r, a, b, c, d.
+	private const XML = '<?xml version="1.0"?><r><i id="a"/><i id="b"/><i id="c"/><i id="d"/></r>';
+
+	public function issueTableProvider(): array
+	{
+		return [
+			'#c :first'        => [['c'], ':first', false],
+			'#c i:first'       => [['c'], 'i:first', false],
+			'#a i:last'        => [['a'], 'i:last', false],
+			'#c, #d i:eq(1)'   => [['c', 'd'], 'i:eq(1)', false],
+			'#a :eq(1)'        => [['a'], ':eq(1)', true],
+			'#c i:eq(2)'       => [['c'], 'i:eq(2)', true],
+			'#c :eq(2)'        => [['c'], ':eq(2)', false],
+			'#c, #d i:last'    => [['c', 'd'], 'i:last', true],
+			'#a, #b i:gt(1)'   => [['a', 'b'], 'i:gt(1)', false],
+			'#a, #d i:odd'     => [['a', 'd'], 'i:odd', true],
+			'#b i:not(:first)' => [['b'], 'i:not(:first)', true],
+			'#a i:not(:first)' => [['a'], 'i:not(:first)', false],
+		];
+	}
+
+	/**
+	 * @dataProvider issueTableProvider
+	 */
+	public function testIsCountsAPositionalSelectorAcrossTheDocument(array $ids, $selector, $expected)
+	{
+		$this->assertSame($expected, $this->selectIds(self::XML, ...$ids)->is($selector));
+	}
+
+	public function testFilterStillCountsWithinTheSet()
+	{
+		$this->assertSame('c', $this->idList($this->selectIds(self::XML, 'c')->filter('i:first')));
+		$this->assertSame('d', $this->idList($this->selectIds(self::XML, 'c', 'd')->filter('i:eq(1)')));
+	}
+
+	public function testNonPositionalSelectorsAreUnchanged()
+	{
+		$qp = $this->selectIds(self::XML, 'b', 'c');
+
+		$this->assertTrue($qp->is('i'));
+		$this->assertTrue($qp->is('#c'));
+		$this->assertTrue($qp->is('r > i:nth-child(2)'));
+		$this->assertFalse($qp->is('#a'));
+		$this->assertFalse($qp->is('r'));
+	}
+
+	public function testNodeAndSetArgumentsAreUnchanged()
+	{
+		$qp = $this->selectIds(self::XML, 'c');
+		$this->assertTrue($qp->is($qp->get(0)));
+		$this->assertFalse($qp->is(qp(self::XML, '#c')->get(0)));
+
+		$set = new SplObjectStorage();
+		$set->offsetSet($qp->get(0));
+		$this->assertTrue($qp->is($set));
+	}
+
+	public function testNodesThatAreNotElementsAreSkipped()
+	{
+		$xml = '<?xml version="1.0"?><r>text<i id="a"/><!-- c --><i id="b"/></r>';
+
+		$this->assertTrue(qp($xml, 'r')->contents()->is('i:first'));
+		$this->assertFalse(qp($xml, 'r')->contents()->is('i:eq(5)'));
+		$this->assertFalse(qp($xml, 'r')->contents()->eq(0)->is(':first'));
+	}
+}