The reduction of CO2 in sulfuric and perchloric acid electrolytes on a polycrystalline Cu surface has been investigated, using the online electrochemical mass spectrometry method (EUMS). Gaseous products, such as CH4, C2H4 and highly volatile products such as CH3OH, HCHO, are formed at overpotentials of hydrogen evolution reaction. The EC/MS results suggest that the reduction products formed depend on the presence of preadsorbed sulfate on the surface. The adsorbed anion from the electrolyte plays a role at the potential where HCHO formation occurs on copper. A mechanism for this process, based on electrocatalytic hydrogenation, is proposed. (C) 2005 Elsevier B.V. All rights reserved.