A new electrochemical sensor (the "redox cycling sensor") for the detection of redox active species like H2S in aqueous solution has been developed. The novel sensor principle uses redox cycling of a redox mediator at electrodes in close proximity (thin-layer cell) to achieve high current signals and high S/N ratios. The sensor cell is separated from the solution by a membrane permeable to the analyte which reacts with the suitable form of the redox mediator contained in excess in the sensor electrolyte. It is possible to use redox cycling for the determination of analyte concentrations without preparing the analyte solution because the redox cycling takes place in the sensor electrolyte not in the analyte solution. In order to measure continuously, the ratio of the forms of the redox mediator is set back to the initial state by a specially designed initialisation circuit. The redox cycling sensor has been constructed with electrodes on glass plates produced by the thin-layer technique and by bonding all parts with an UV-A hardening adhesive. The distance between the electrodes (< 10 mu m) has been realised by etching a channel into the lower glass plate before sputtering the electrodes. The redox cycling sensor was calibrated for H2S in a flow system combined with a coulometric H2S generator reaching a detection limit of about 40 nmol L-1. (c) 2007 Elsevier B.V. All rights reserved.