diff --git a/src/Http/Response.php b/src/Http/Response.php
index 4d4485e3..1500d2da 100644
--- a/src/Http/Response.php
+++ b/src/Http/Response.php
@@ -6,7 +6,10 @@
 
 use Throwable;
 use LogicException;
+use function implode;
 use SimpleXMLElement;
+use function is_array;
+use function mb_strtolower;
 use Saloon\Traits\Macroable;
 use InvalidArgumentException;
 use Saloon\Helpers\ArrayHelpers;
@@ -486,7 +489,13 @@ public function throw(): static
      */
     public function header(string $header): string|array|null
     {
-        return $this->headers()->get($header);
+        if (! $this->psrResponse->hasHeader($header)) {
+            return null;
+        }
+
+        $values = $this->psrResponse->getHeader($header);
+
+        return count($values) === 1 ? $values[0] : $values;
     }
 
     /**
@@ -494,13 +503,17 @@ public function header(string $header): string|array|null
      */
     public function isJson(): bool
     {
-        $contentType = $this->psrResponse->getHeaderLine('Content-Type');
+        $contentType = $this->header('Content-Type');
 
-        if ($contentType === '') {
+        if (empty($contentType)) {
             return false;
         }
 
-        return str_contains($contentType, 'json');
+        if (is_array($contentType)) {
+            $contentType = implode(',', $contentType);
+        }
+
+        return str_contains(mb_strtolower($contentType), 'json');
     }
 
     /**
@@ -508,13 +521,17 @@ public function isJson(): bool
      */
     public function isXml(): bool
     {
-        $contentType = $this->psrResponse->getHeaderLine('Content-Type');
+        $contentType = $this->header('Content-Type');
 
-        if ($contentType === '') {
+        if (empty($contentType)) {
             return false;
         }
 
-        return str_contains($contentType, 'xml');
+        if (is_array($contentType)) {
+            $contentType = implode(',', $contentType);
+        }
+
+        return str_contains(mb_strtolower($contentType), 'xml');
     }
 
     /**
diff --git a/tests/Unit/ResponseTest.php b/tests/Unit/ResponseTest.php
index 02d0e363..95fbc452 100644
--- a/tests/Unit/ResponseTest.php
+++ b/tests/Unit/ResponseTest.php
@@ -472,3 +472,36 @@
     $response = $connector->send(new UserRequest, $mockClient);
     expect($response->isXml())->toBeFalse();
 });
+
+test('header lookup is case-insensitive per HTTP RFC', function () {
+    $mockClient = new MockClient([
+        MockResponse::make([], 200, ['x-my-custom-header' => 'custom-value']),
+    ]);
+
+    $response = connector()->send(new UserRequest, $mockClient);
+
+    expect($response->header('X-My-Custom-Header'))->toEqual('custom-value');
+    expect($response->header('x-my-custom-header'))->toEqual('custom-value');
+});
+
+test('isJson lookup can use case insensitive headers', function () {
+    $mockClient = new MockClient([
+        MockResponse::make([], 200, ['content-type' => 'application/JSON']),
+    ]);
+
+    $response = connector()->send(new UserRequest, $mockClient);
+
+    expect($response->isJson())->toBeTrue();
+    expect($response->isXml())->toBeFalse();
+});
+
+test('isXml lookup can use case insensitive headers', function () {
+    $mockClient = new MockClient([
+        MockResponse::make([], 200, ['content-type' => 'text/xml']),
+    ]);
+
+    $response = connector()->send(new UserRequest, $mockClient);
+
+    expect($response->isXml())->toBeTrue();
+    expect($response->isJson())->toBeFalse();
+});