Here we report another surprising feature of the methylammonium metal halide material family, the phototunability of the diode response of a heterojunction made of CH3NH3PbI3 and its close relative, CH3NH3SnI3. In the dark state the device behaves as a diode, with the Sn homologue acting as the "p" side. The junction is extremely sensitive to illumination. A complete reversal of the diode polarity, the first observation of its kind, is seen when the junction is exposed to red laser light of 25 mW/cm(2) or larger power density. This finding opens up the possibility for a novel class of optoelectronic devices.