A new amperometric microsensor for detection of dissolved H₂S in aquatic environments was developed. The design of the microsensor is based on the same principle as the dark-type oxygen microsensor. The sensor is equipped with a glass-coated platinum working electrode and a platinum guard electrode positioned in an outer glass casing (tip diameter 20-100 μm). Both working electrode and guard electrode were polarized at a fixed value in the range from +85 to + 150 mV with respect to a counter electrode. The outer casing is sealed with a thin silicone membrane and filled with a buffered electrolyte solution containing ferricyanide (K₃[Fe(CN)₆]) as redox mediator. Hydrogen sulfide penetrates the silicone membrane and is oxidized by K₃[Fe(CN)₆], resulting in the formation of elemental sulfur and ferrocyanide (K₄-[Fe(CN)₆]). The latter is electrochemically reoxidized at the exposed end of the platinum working electrode, thereby creating a current that is directly proportional to the dissolved H₂S concentration at the sensor tip. The sensor was characterized and calibrated in a flow-through cell combined with a coulometric sulfide generator. Difficult studies including the determination of H₂S with high spatial and temporal resolution seem to be possible.