diff --git a/Lib/test/test_hashlib.py b/Lib/test/test_hashlib.py
index 90e6a563a72..e4e5280dc8e 100644
--- a/Lib/test/test_hashlib.py
+++ b/Lib/test/test_hashlib.py
@@ -615,6 +615,24 @@ class HashLibTestCase(unittest.TestCase):
           "ba80a53f981c4d0d6a2797b69f12f6e94c212f14685ac4b74b12bb6fdbffa2d1"+
           "7d87c5392aab792dc252d5de4533cc9518d38aa8dbf1925ab92386edd4009923")
 
+    @requires_blake2
+    def test_case_blake2b_all_parameters(self):
+        # This checks that all the parameters work in general, and also that
+        # parameter byte order doesn't get confused on big endian platforms.
+        self.check('blake2b', b"foo",
+          "920568b0c5873b2f0ab67bedb6cf1b2b",
+          digest_size=16,
+          key=b"bar",
+          salt=b"baz",
+          person=b"bing",
+          fanout=2,
+          depth=3,
+          leaf_size=4,
+          node_offset=5,
+          node_depth=6,
+          inner_size=7,
+          last_node=True)
+
     @requires_blake2
     def test_blake2b_vectors(self):
         for msg, key, md in read_vectors('blake2b'):
@@ -640,6 +658,24 @@ class HashLibTestCase(unittest.TestCase):
         self.check('blake2s', b"abc",
           "508c5e8c327c14e2e1a72ba34eeb452f37458b209ed63a294d999b4c86675982")
 
+    @requires_blake2
+    def test_case_blake2s_all_parameters(self):
+        # This checks that all the parameters work in general, and also that
+        # parameter byte order doesn't get confused on big endian platforms.
+        self.check('blake2s', b"foo",
+          "bf2a8f7fe3c555012a6f8046e646bc75",
+          digest_size=16,
+          key=b"bar",
+          salt=b"baz",
+          person=b"bing",
+          fanout=2,
+          depth=3,
+          leaf_size=4,
+          node_offset=5,
+          node_depth=6,
+          inner_size=7,
+          last_node=True)
+
     @requires_blake2
     def test_blake2s_vectors(self):
         for msg, key, md in read_vectors('blake2s'):
diff --git a/Misc/NEWS.d/next/Library/2017-11-03-08-36-03.bpo-31933.UrtoMP.rst b/Misc/NEWS.d/next/Library/2017-11-03-08-36-03.bpo-31933.UrtoMP.rst
new file mode 100644
index 00000000000..c4ee1d24980
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-11-03-08-36-03.bpo-31933.UrtoMP.rst
@@ -0,0 +1,2 @@
+Fix Blake2 params leaf_size and node_offset on big endian platforms. Patch
+by Jack O'Connor.
diff --git a/Modules/_blake2/blake2b_impl.c b/Modules/_blake2/blake2b_impl.c
index 3c2a035f3d7..822b9f285ca 100644
--- a/Modules/_blake2/blake2b_impl.c
+++ b/Modules/_blake2/blake2b_impl.c
@@ -162,7 +162,8 @@ py_blake2b_new_impl(PyTypeObject *type, PyObject *data, int digest_size,
             goto error;
         }
     }
-    self->param.leaf_length = (unsigned int)leaf_size;
+    // NB: Simple assignment here would be incorrect on big endian platforms.
+    store32(&(self->param.leaf_length), leaf_size);
 
     if (node_offset_obj != NULL) {
         node_offset = PyLong_AsUnsignedLongLong(node_offset_obj);
@@ -178,7 +179,8 @@ py_blake2b_new_impl(PyTypeObject *type, PyObject *data, int digest_size,
      }
     store48(&(self->param.node_offset), node_offset);
 #else
-    self->param.node_offset = node_offset;
+    // NB: Simple assignment here would be incorrect on big endian platforms.
+    store64(&(self->param.node_offset), node_offset);
 #endif
 
     if (node_depth < 0 || node_depth > 255) {
diff --git a/Modules/_blake2/blake2s_impl.c b/Modules/_blake2/blake2s_impl.c
index 2c5697299fc..3b76c9cd89d 100644
--- a/Modules/_blake2/blake2s_impl.c
+++ b/Modules/_blake2/blake2s_impl.c
@@ -162,7 +162,8 @@ py_blake2s_new_impl(PyTypeObject *type, PyObject *data, int digest_size,
             goto error;
         }
     }
-    self->param.leaf_length = (unsigned int)leaf_size;
+    // NB: Simple assignment here would be incorrect on big endian platforms.
+    store32(&(self->param.leaf_length), leaf_size);
 
     if (node_offset_obj != NULL) {
         node_offset = PyLong_AsUnsignedLongLong(node_offset_obj);
@@ -178,7 +179,8 @@ py_blake2s_new_impl(PyTypeObject *type, PyObject *data, int digest_size,
      }
     store48(&(self->param.node_offset), node_offset);
 #else
-    self->param.node_offset = node_offset;
+    // NB: Simple assignment here would be incorrect on big endian platforms.
+    store64(&(self->param.node_offset), node_offset);
 #endif
 
     if (node_depth < 0 || node_depth > 255) {