The present invention provides a system for the determination of an analyte in a liquid medium. The system comprises a self-powered biosensor and a detector for measuring an electrical signal generated by said biosensor while the analyte is being oxidized or reduced, the biosensor comprising a pair of electrodes, one of the electrodes being an anode and the other a cathode, both of which carry redox enzymes on their surface. An enzyme carried on one of the electrodes can catalyze an oxidation or reduction reaction in which the analyte is oxidized or reduced, respectively, and the other of said pair of electrodes carries on its surface enzymes that can catalyze a reaction in which the oxidizer or the reducer are reduced or oxidized, respectively, in the presence of the analyte.