diff --git a/composer.json b/composer.json
index 69f5da0..35b4a57 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,7 @@
         "php" : ">=5.3.0"
     },
     "require-dev": {
-        "phpunit/phpunit" : "^4.8.35 || ^5.7.21 || ^6 || ^7 || ^8"
+        "phpunit/phpunit" : "^4.8.35 || ^5.7.21 || ^6 || ^7 || ^8 || ^9"
     },
     "autoload": {
         "psr-4": {"Masterminds\\": "src"}
diff --git a/test/HTML5/Html5Test.php b/test/HTML5/Html5Test.php
index 62f36e1..7c2f485 100644
--- a/test/HTML5/Html5Test.php
+++ b/test/HTML5/Html5Test.php
@@ -147,7 +147,12 @@ public function testEncodingUtf8()
         $this->assertEmpty($this->html5->getErrors());
         $this->assertFalse($this->html5->hasErrors());
 
-        $this->assertContains('Žťčýů', $dom->saveHTML());
+        // phpunit 9
+        if (method_exists($this, 'assertStringContainsString')) {
+            $this->assertStringContainsString('Žťčýů', $dom->saveHTML());
+        } else {
+            $this->assertContains('Žťčýů', $dom->saveHTML());
+        }
     }
 
     public function testEncodingWindows1252()
@@ -427,7 +432,13 @@ public function testElements()
     public function testAttributes()
     {
         $res = $this->cycle('<use xlink:href="#svg-track" xmlns:xlink="http://www.w3.org/1999/xlink"></use>');
-        $this->assertContains('<use xlink:href="#svg-track" xmlns:xlink="http://www.w3.org/1999/xlink"></use>', $res);
+
+        // phpunit 9
+        if (method_exists($this, 'assertStringContainsString')) {
+            $this->assertStringContainsString('<use xlink:href="#svg-track" xmlns:xlink="http://www.w3.org/1999/xlink"></use>', $res);
+        } else {
+            $this->assertContains('<use xlink:href="#svg-track" xmlns:xlink="http://www.w3.org/1999/xlink"></use>', $res);
+        }
 
         $res = $this->cycle('<div attr="val">FOO</div>');
         $this->assertRegExp('|<div attr="val">FOO</div>|', $res);
@@ -541,9 +552,12 @@ public function testCDATA()
     public function testAnchorTargetQueryParam()
     {
         $res = $this->cycle('<a href="https://domain.com/page.php?foo=bar&target=baz">https://domain.com/page.php?foo=bar&target=baz</a>');
-        $this->assertContains(
-            '<a href="https://domain.com/page.php?foo=bar&amp;target=baz">https://domain.com/page.php?foo=bar&amp;target=baz</a>',
-            $res
-        );
+
+        // phpunit 9
+        if (method_exists($this, 'assertStringContainsString')) {
+            $this->assertStringContainsString('<a href="https://domain.com/page.php?foo=bar&amp;target=baz">https://domain.com/page.php?foo=bar&amp;target=baz</a>', $res);
+        } else {
+            $this->assertContains('<a href="https://domain.com/page.php?foo=bar&amp;target=baz">https://domain.com/page.php?foo=bar&amp;target=baz</a>', $res);
+        }
     }
 }