Photovoltaic tandem cells, consisting of a gallium indium phosphide (GaInP2) homojunction grown epitaxially on a gallium arsenide (GaAs) homojunction with a GaAs tunnel diode interconnect, were modified with an additional top player of GaInP2. These cells were used as electrodes to photoelectrochemically decompose water into hydrogen and oxygen in 1M, 5M and 11M KOH electrolyte solutions. The hydrogen reaction was catalyzed at the semiconductor surface with a photoelectrochemically deposited thin layer of platinum and ruthenium. Gas chromatography and electrochemical experiments demonstrate that the modified tandem cells produce hydrogen and oxygen with a light-to-hydrogen conversion efficiency of up to 6%. Both the efficiency and the stability of these cells are discussed.