Photovoltaic tandem cells consisting of a gallium indium phosphide (GaInP2) homojunction grown epitaxially upon a gallium arsenide (GaAs) homojunction, with a GaAs tunnel-diode interconnection were utilized to photoelectrochemically decompose water in a 1 M sulfuric acid electrolyte solution. Using a sol-gel process, a platinum colloid in water was used to modify the illuminated front surface of the device to catalyze the water decomposition process. A unique feature of this device is that the hydrogen and oxygen are co-evolved from the illuminated surface. The exact mechanism of the water decomposition process is not fully understood at this time.