In an effort to find a semiconductor electrode with a bandgap in the visible part of the spectrum that can serve in the catalytic photodecomposition of water, different systems were explored. The first was sputtered thin films of wide bandgap semiconductor materials such as TiO2, SnO2, Nb2O3, AI2O3, and Si3N4 on low bandgap, n-type semiconductors such as GaAs and GaAlAs. Scanning electron micrographs showed that corrosion is greatly reduced but continues by diffusion of the electrolyte through the film, undermining the film by pit formation in the low bandgap semiconductor. There was no evidence for hole conduction through the film. The second system employed p-type GaP as the cathode and Pt as the anode. It was observed that this cell catalytically photoelectrolyzes water with conversion efficiency of 0.1%. The efficiency of the device is a strong function of crystal orientation, surface treatment, and purity of the crystal. The results are explained in terms of an energy diagram of the entire system. Areas for possible improvement are mentioned. © 1977, The Electrochemical Society, Inc. All rights reserved.