diff --git a/jax/flatten_util.py b/jax/flatten_util.py
index 7324de1a0fa2..249ff0441587 100644
--- a/jax/flatten_util.py
+++ b/jax/flatten_util.py
@@ -12,12 +12,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import warnings
+
+import numpy as np
 
 from .tree_util import tree_flatten, tree_unflatten
-from ._src.util import safe_zip
+from ._src.util import safe_zip, unzip2
 
 import jax.numpy as jnp
-from jax.api import vjp
+from jax import dtypes
+from jax import lax
 
 zip = safe_zip
 
@@ -26,18 +30,40 @@ def ravel_pytree(pytree):
   """Ravel (i.e. flatten) a pytree of arrays down to a 1D array.
 
   Args:
-    pytree: a pytree to ravel.
+    pytree: a pytree of arrays and scalars to ravel.
 
   Returns:
     A pair where the first element is a 1D array representing the flattened and
-    concatenated leaf values, and the second element is a callable for
-    unflattening a 1D vector of the same length back to a pytree of of the same
-    structure as the input ``pytree``.
+    concatenated leaf values, with dtype determined by promoting the dtypes of
+    leaf values, and the second element is a callable for unflattening a 1D
+    vector of the same length back to a pytree of of the same structure as the
+    input ``pytree``. If the input pytree is empty (i.e. has no leaves) then as
+    a convention a 1D empty array of dtype float32 is returned in the first
+    component of the output.
+
+  For details on dtype promotion, see
+  https://jax.readthedocs.io/en/latest/type_promotion.html.
+
   """
   leaves, treedef = tree_flatten(pytree)
-  flat, unravel_list = vjp(_ravel_list, *leaves)
+  flat, unravel_list = _ravel_list(leaves)
   unravel_pytree = lambda flat: tree_unflatten(treedef, unravel_list(flat))
   return flat, unravel_pytree
 
-def _ravel_list(*lst):
-  return jnp.concatenate([jnp.ravel(elt) for elt in lst]) if lst else jnp.array([])
+def _ravel_list(lst):
+  if not lst: return jnp.array([], jnp.float32), lambda _: []
+  from_dtypes = [dtypes.dtype(l) for l in lst]
+  to_dtype = dtypes.result_type(*from_dtypes)
+  sizes, shapes = unzip2((jnp.size(x), jnp.shape(x)) for x in lst)
+  indices = np.cumsum(sizes)
+
+  def unravel(arr):
+    chunks = jnp.split(arr, indices[:-1])
+    with warnings.catch_warnings():
+      warnings.simplefilter("ignore")  # ignore complex-to-real cast warning
+      return [lax.convert_element_type(chunk.reshape(shape), dtype)
+              for chunk, shape, dtype in zip(chunks, shapes, from_dtypes)]
+
+  ravel = lambda e: jnp.ravel(lax.convert_element_type(e, to_dtype))
+  raveled = jnp.concatenate([ravel(e) for e in lst])
+  return raveled, unravel
diff --git a/tests/tree_util_test.py b/tests/tree_util_test.py
index 6e8ed3d501d5..52f026da0169 100644
--- a/tests/tree_util_test.py
+++ b/tests/tree_util_test.py
@@ -20,6 +20,9 @@
 
 from jax import test_util as jtu
 from jax import tree_util
+from jax import flatten_util
+from jax import dtypes
+import jax.numpy as jnp
 
 
 def _dummy_func(*args, **kwargs):
@@ -274,5 +277,56 @@ def testTransposeWithCustomObject(self):
                                       FlatCache({"a": [3, 4], "b": [5, 6]}))
     self.assertEqual(expected, actual)
 
+
+class RavelUtilTest(jtu.JaxTestCase):
+
+  def testFloats(self):
+    tree = [jnp.array([3.], jnp.float32),
+            jnp.array([[1., 2.], [3., 4.]], jnp.float32)]
+    raveled, unravel = flatten_util.ravel_pytree(tree)
+    self.assertEqual(raveled.dtype, jnp.float32)
+    tree_ = unravel(raveled)
+    self.assertAllClose(tree, tree_, atol=0., rtol=0.)
+
+  def testInts(self):
+    tree = [jnp.array([3], jnp.int32),
+            jnp.array([[1, 2], [3, 4]], jnp.int32)]
+    raveled, unravel = flatten_util.ravel_pytree(tree)
+    self.assertEqual(raveled.dtype, jnp.int32)
+    tree_ = unravel(raveled)
+    self.assertAllClose(tree, tree_, atol=0., rtol=0.)
+
+  def testMixedFloatInt(self):
+    tree = [jnp.array([3], jnp.int32),
+            jnp.array([[1., 2.], [3., 4.]], jnp.float32)]
+    raveled, unravel = flatten_util.ravel_pytree(tree)
+    self.assertEqual(raveled.dtype, dtypes.promote_types(jnp.float32, jnp.int32))
+    tree_ = unravel(raveled)
+    self.assertAllClose(tree, tree_, atol=0., rtol=0.)
+
+  def testMixedIntBool(self):
+    tree = [jnp.array([0], jnp.bool_),
+            jnp.array([[1, 2], [3, 4]], jnp.int32)]
+    raveled, unravel = flatten_util.ravel_pytree(tree)
+    self.assertEqual(raveled.dtype, dtypes.promote_types(jnp.bool_, jnp.int32))
+    tree_ = unravel(raveled)
+    self.assertAllClose(tree, tree_, atol=0., rtol=0.)
+
+  def testMixedFloatComplex(self):
+    tree = [jnp.array([1.], jnp.float32),
+            jnp.array([[1, 2 + 3j], [3, 4]], jnp.complex64)]
+    raveled, unravel = flatten_util.ravel_pytree(tree)
+    self.assertEqual(raveled.dtype, dtypes.promote_types(jnp.float32, jnp.complex64))
+    tree_ = unravel(raveled)
+    self.assertAllClose(tree, tree_, atol=0., rtol=0.)
+
+  def testEmpty(self):
+    tree = []
+    raveled, unravel = flatten_util.ravel_pytree(tree)
+    self.assertEqual(raveled.dtype, jnp.float32)  # convention
+    tree_ = unravel(raveled)
+    self.assertAllClose(tree, tree_, atol=0., rtol=0.)
+
+
 if __name__ == "__main__":
   absltest.main(testLoader=jtu.JaxTestLoader())