A reaction is performed in a micro-reactor (10), voltages being applied by a power supply (12), under the control of a processor (14), so as to cause the reagents to move and react in the channels (24,30,32,34,36) of the micro-reactor (10). The power supply (12) also measures currents in the channels and relays this information to the processor (14). The currents are used to monitor progress of the reaction.