We studied H-2 gas generation using a GaN photoelectrode in both HCl and KOH solutions. The photocurrent in KOH was larger than that in HCl under a small extra bias, and caused a more extensive H-2 generation at the beginning. GaN corrosion in HCl was less extended than that in KOH during photoelectrochemical reactions. The stability of the GaN photoelectrode in HCl was a result of H2O reduction and Cl- oxidation instead of water photoelectrolysis. (c) 2006 The Electrochemical Society.