Geobacter sulfurreducens is a well-known current-producing microorganism in microbial fuel cells, and is able to use acetate and hydrogen as electron donor. We studied the functionality of G. sulfurreducens as biocatalyst for hydrogen formation at the cathode of a microbial electrolysis cell (MEC). Geobacter sulfurreducens was grown in the bioelectrode compartment of a MFC with acetate as the substrate and reduction of complexed Fe(III) at the counter electrode. After depletion of the acetate the electrode potential of the bioelectrode was decreased stepwise to -1.0 V vs Ag/AgCl reference. Production of negative current was observed, which increased in time, indicating that the bioelectrode was now acting as biocathode. Headspace analyses carried out at electrode potentials ranging from -0.8 to -1.0 V showed that hydrogen was produced, with higher rates at more negative cathode potentials. Subsequently, the metabolic properties of G. sulfurreducens for acetate oxidation at the anode and hydrogen production at the cathode were combined in one-compartment membraneless MECs operated at applied voltages of 0.8 and 0.65 V. After two days, current densities were 0.44 A m(-2) at 0.8 V applied voltage and 0.22 A m(-2) at 0.65 V, using flat-surface carbon electrodes for both anode and cathode. The cathodic hydrogen recovery ranged from 23% at 0.5 V applied voltage to 43% at 0.9 V.