The electrochemical step in two thermochemical cycles for hydrogen production is reported. One cycle involves the electrochemical oxidation of sulfur dioxide to sulfuric acid (both water and SO2 are reactants). The other cycle involves the oxidation of anhydrous hydrogen bromide to bromine (anhydrous HBr is the only reactant). In both cycles, protons are reduced at the cathode to produce hydrogen. The novelty of this work is that both anode reactions are carried out in the gas phase of a proton exchange membrane (PEM) electrolyzer, which enhances the transport rate of reactants to the electrode surface. The HBr process achieved 2.0 A/cm(2) at 1.91 V. The SO2 process reached 0.4 A/cm(2), but behind this current density the cell experienced mass transfer limitations of water across the membrane. However, the voltage required to achieve 0.4A/cm(2) was 0.835V, compared to 1.025V for the HBr process at this current density. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.