This paper presents a description and analysis of a novel, high-efficiency, solid oxide natural gas-assisted steam electrolyzer (NGASE). In conventional solid oxide electrolyzers, most of the electric power is used in forcing the oxygen to diffuse through the electrolyzer, against a high chemical potential. In the NGASE, natural gas is reacted with the oxygen produced in the electrolysis, reducing the chemical potential across the electrolyzer, thus minimizing electricity consumption. The oxygen produced in the electrolysis is consumed in either a total oxidation or a partial oxidation reaction with natural gas. Experiments performed on single cells show a voltage reduction of as much as 1 V when compared to conventional steam electrolyzers. Analysis indicates that incorporating the electrolyzer with a heat recovery system (heat exchangers and catalytic reactor) results in a high-efficiency hydrogen production system. The system efficiency is up to 70% with respect to primary energy. (C) 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.