diff --git a/Tests/test_file_container.py b/Tests/test_file_container.py
index 91166b39e72..b752e217faa 100644
--- a/Tests/test_file_container.py
+++ b/Tests/test_file_container.py
@@ -20,7 +20,7 @@ def test_isatty():
 def test_seek_mode_0():
     # Arrange
     mode = 0
-    with open(TEST_FILE) as fh:
+    with open(TEST_FILE, "rb") as fh:
         container = ContainerIO.ContainerIO(fh, 22, 100)
 
         # Act
@@ -34,7 +34,7 @@ def test_seek_mode_0():
 def test_seek_mode_1():
     # Arrange
     mode = 1
-    with open(TEST_FILE) as fh:
+    with open(TEST_FILE, "rb") as fh:
         container = ContainerIO.ContainerIO(fh, 22, 100)
 
         # Act
@@ -48,7 +48,7 @@ def test_seek_mode_1():
 def test_seek_mode_2():
     # Arrange
     mode = 2
-    with open(TEST_FILE) as fh:
+    with open(TEST_FILE, "rb") as fh:
         container = ContainerIO.ContainerIO(fh, 22, 100)
 
         # Act
@@ -61,73 +61,87 @@ def test_seek_mode_2():
 
 def test_read_n0():
     # Arrange
-    with open(TEST_FILE) as fh:
-        container = ContainerIO.ContainerIO(fh, 22, 100)
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+            container = ContainerIO.ContainerIO(fh, 22, 100)
 
-        # Act
-        container.seek(81)
-        data = container.read()
+            # Act
+            container.seek(81)
+            data = container.read()
 
-        # Assert
-        assert data == "7\nThis is line 8\n"
+            # Assert
+            if bytesmode:
+                data = data.decode()
+            assert data == "7\nThis is line 8\n"
 
 
 def test_read_n():
     # Arrange
-    with open(TEST_FILE) as fh:
-        container = ContainerIO.ContainerIO(fh, 22, 100)
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+            container = ContainerIO.ContainerIO(fh, 22, 100)
 
-        # Act
-        container.seek(81)
-        data = container.read(3)
+            # Act
+            container.seek(81)
+            data = container.read(3)
 
-        # Assert
-        assert data == "7\nT"
+            # Assert
+            if bytesmode:
+                data = data.decode()
+            assert data == "7\nT"
 
 
 def test_read_eof():
     # Arrange
-    with open(TEST_FILE) as fh:
-        container = ContainerIO.ContainerIO(fh, 22, 100)
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+            container = ContainerIO.ContainerIO(fh, 22, 100)
 
-        # Act
-        container.seek(100)
-        data = container.read()
+            # Act
+            container.seek(100)
+            data = container.read()
 
-        # Assert
-        assert data == ""
+            # Assert
+            if bytesmode:
+                data = data.decode()
+            assert data == ""
 
 
 def test_readline():
     # Arrange
-    with open(TEST_FILE) as fh:
-        container = ContainerIO.ContainerIO(fh, 0, 120)
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+            container = ContainerIO.ContainerIO(fh, 0, 120)
 
-        # Act
-        data = container.readline()
+            # Act
+            data = container.readline()
 
-        # Assert
-        assert data == "This is line 1\n"
+            # Assert
+            if bytesmode:
+                data = data.decode()
+            assert data == "This is line 1\n"
 
 
 def test_readlines():
     # Arrange
-    expected = [
-        "This is line 1\n",
-        "This is line 2\n",
-        "This is line 3\n",
-        "This is line 4\n",
-        "This is line 5\n",
-        "This is line 6\n",
-        "This is line 7\n",
-        "This is line 8\n",
-    ]
-    with open(TEST_FILE) as fh:
-        container = ContainerIO.ContainerIO(fh, 0, 120)
-
-        # Act
-        data = container.readlines()
-
-        # Assert
-
-        assert data == expected
+    for bytesmode in (True, False):
+        expected = [
+            "This is line 1\n",
+            "This is line 2\n",
+            "This is line 3\n",
+            "This is line 4\n",
+            "This is line 5\n",
+            "This is line 6\n",
+            "This is line 7\n",
+            "This is line 8\n",
+        ]
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+            container = ContainerIO.ContainerIO(fh, 0, 120)
+
+            # Act
+            data = container.readlines()
+
+            # Assert
+            if bytesmode:
+                data = [line.decode() for line in data]
+            assert data == expected
diff --git a/src/PIL/ContainerIO.py b/src/PIL/ContainerIO.py
index 9727601abc3..5bb0086f6e7 100644
--- a/src/PIL/ContainerIO.py
+++ b/src/PIL/ContainerIO.py
@@ -82,7 +82,7 @@ def read(self, n=0):
         else:
             n = self.length - self.pos
         if not n:  # EOF
-            return ""
+            return b"" if "b" in self.fh.mode else ""
         self.pos = self.pos + n
         return self.fh.read(n)
 
@@ -92,13 +92,14 @@ def readline(self):
 
         :returns: An 8-bit string.
         """
-        s = ""
+        s = b"" if "b" in self.fh.mode else ""
+        newline_character = b"\n" if "b" in self.fh.mode else "\n"
         while True:
             c = self.read(1)
             if not c:
                 break
             s = s + c
-            if c == "\n":
+            if c == newline_character:
                 break
         return s