diff --git a/pandas/core/common.py b/pandas/core/common.py
index 586f36cbd0b0f..28f3a19ab5298 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -754,17 +754,36 @@ def _maybe_upcast_putmask(result, mask, other, dtype=None, change=None):
     if mask.any():
 
         def changeit():
+
             # our type is wrong here, need to upcast
-            if (-mask).any():
-                r, fill_value = _maybe_upcast(result, fill_value=other, dtype=dtype, copy=True)
-                np.putmask(r, mask, other)
+            r, fill_value = _maybe_upcast(result, fill_value=other, dtype=dtype, copy=True)
+            np.putmask(r, mask, other)
+                
+            # we need to actually change the dtype here
+            if change is not None:
+                change.dtype = r.dtype
+                change[:] = r
                 
-                # we need to actually change the dtype here
-                if change is not None:
-                    change.dtype = r.dtype
-                    change[:] = r
+            return r, True
+
+        # we want to decide whether putmask will work
+        # if we have nans in the False portion of our mask then we need to upcast (possibily)
+        # otherwise we DON't want to upcast (e.g. if we are have values, say integers in
+        # the success portion then its ok to not upcast)
+        new_dtype, fill_value = _maybe_promote(result.dtype,other)
+        if new_dtype != result.dtype:
+
+            # we have a scalar or len 0 ndarray 
+            # and its nan and we are changing some values
+            if np.isscalar(other) or (isinstance(other,np.ndarray) and other.ndim < 1):
+                if isnull(other):
+                    return changeit()
+
+            # we have an ndarray and the masking has nans in it
+            else:
 
-                return r, True
+                if isnull(other[mask]).any():
+                    return changeit()
 
         try:
             np.putmask(result, mask, other)
@@ -811,9 +830,9 @@ def _possibly_downcast_to_dtype(result, dtype):
         return result
 
     try:
-        if dtype == np.float_:
+        if issubclass(dtype.type,np.floating):
             return result.astype(dtype)
-        elif dtype == np.bool_ or dtype == np.int_:
+        elif dtype == np.bool_ or issubclass(dtype.type,np.integer):
             if issubclass(result.dtype.type, np.number) and notnull(result).all():
                 new_result = result.astype(dtype)
                 if (new_result == result).all():
diff --git a/pandas/core/index.py b/pandas/core/index.py
index 467fe46e81efd..0d65577546f8b 100644
--- a/pandas/core/index.py
+++ b/pandas/core/index.py
@@ -107,7 +107,7 @@ def __new__(cls, data, dtype=None, copy=False, name=None):
                 return PeriodIndex(data, copy=copy, name=name)
 
             if issubclass(data.dtype.type, np.integer):
-                return Int64Index(data, copy=copy, name=name)
+                return Int64Index(data, copy=copy, dtype=dtype, name=name)
 
             subarr = com._ensure_object(data)
         elif np.isscalar(data):
@@ -1296,7 +1296,12 @@ def __new__(cls, data, dtype=None, copy=False, name=None):
             raise TypeError('String dtype not supported, you may need '
                             'to explicitly cast to int')
         elif issubclass(data.dtype.type, np.integer):
-            subarr = np.array(data, dtype=np.int64, copy=copy)
+            # don't force the upcast as we may be dealing
+            # with a platform int
+            if dtype is None or not issubclass(np.dtype(dtype).type, np.integer):
+                dtype = np.int64
+
+            subarr = np.array(data, dtype=dtype, copy=copy)
         else:
             subarr = np.array(data, dtype=np.int64, copy=copy)
             if len(data) > 0:
@@ -1316,10 +1321,6 @@ def inferred_type(self):
     def _constructor(self):
         return Int64Index
 
-    @cache_readonly
-    def dtype(self):
-        return np.dtype('int64')
-
     @property
     def is_all_dates(self):
         """
diff --git a/pandas/tests/test_common.py b/pandas/tests/test_common.py
index 7869d2627d581..89f6859a39bb0 100644
--- a/pandas/tests/test_common.py
+++ b/pandas/tests/test_common.py
@@ -253,6 +253,26 @@ def test_ensure_int32():
     result = com._ensure_int32(values)
     assert(result.dtype == np.int32)
 
+def test_ensure_platform_int():
+
+    # verify that when we create certain types of indices
+    # they remain the correct type under platform conversions
+    from pandas.core.index import Int64Index
+
+    # int64
+    x = Int64Index([1, 2, 3], dtype='int64')
+    assert(x.dtype == np.int64)
+
+    pi = com._ensure_platform_int(x)
+    assert(pi.dtype == np.int_)
+
+    # int32
+    x = Int64Index([1, 2, 3], dtype='int32')
+    assert(x.dtype == np.int32)
+
+    pi = com._ensure_platform_int(x)
+    assert(pi.dtype == np.int_)
+
 # TODO: fix this broken test
 
 # def test_console_encode():