Ok, so you guys shot down my last attempt at finding a bug :). Here's another attempt. array + masked_array outputs a masked array array += masked_array outputs an array. I'm actually not sure if this is a bug (works the same for both the old and new masked arrays), but I thought this was unexpected. *However*, if the right side were an old masked array, the masked values would not be added to the output array. With the new masked arrays, the data from the masked array is added regardless of whether the value is masked (example, below). -Tony Example: ======= In [1]: import numpy In [2]: normal = numpy.array([1, 1]) In [3]: masked = numpy.ma.array([1, 1], mask=[True, False]) In [4]: new_masked = normal + masked In [5]: new_masked Out[5]: masked_array(data = [-- 2], mask = [ True False], fill_value=999999) In [6]: normal += masked In [7]: normal Out[7]: array([2, 2]) # If used old masked arrays in the above, the final output would be: array([1, 2])