H2S and H2Se have been electrolytically prepared by electrolysis in aqueous H2SO4 solutions of composite cathodes made of S or Se and graphite. The efficiencies depended strongly on the electrolyte composition, in particular on the acid concentration and presence of K+. Faradaic efficiencies of 80% were obtained in dilute (0.05 M) acid, and this increased to 100% with added K2SO4. The efficiencies dropped drastically in concentrated acid (> a few moles). H2Te and AsH3 generation were also briefly studied for comparison. The mechanisms of hydride formation are discussed. Both the reaction of nascent hydrogen with the free element and direct reduction of the element are considered. The latter is believed to be the dominant mechanism. (c) 2005 The Electrochemical Society. All rights reserved.