Power production characteristics and substrate concentration dependence of voltage have been investigated together with the determination of kinetic constants in two-chambered mediator-less microbial fuel cells (MFC) for acetate and peptone substrates. At 500 mg DOC l(-1) (dissolved organic carbon), power densities normalized to the anode surface of 112 mW m(-2) with acetate and 114 mW m(-2) with peptone as electron donor were attained by applying cathodes with a Pt catalyst layer. Related anode surface specific substrate removal rate was 44 g DOC m(-2) h(-1) for acetate and 52 g DOC m(-2) h(-1) for peptone. Substrate concentration dependency of the voltage suggests Monod-like kinetics with extremely low, < 1 mg DOC l(-1), half saturation constants and with final DOC concentrations of 6-10 mg l(-1). Acetate and peptone are equivalent substrates for the exoelectrogenic bacteria both from the point of view of biodegradation kinetics and power production characteristics.