We developed a microsensor for the amperometric detection of dissolved hydrogen sulfide, H,S, in sedirnents and biofilms. The mlcrosensor exhibits a fast (tgO c0.2 to 0.5 S ) and linear response to H2S over a concentration range of 1 to >l000 pm01 H2S I-', and has a low stirring dependency of the microsensor signal (c1 to 2%). We used the new mlcrosensor to obtain the first microprofiles of H2S in an acidic lake sediment with a several cm thick flocculant surface layer. Despite the low pH of 4.6, a relative low SO,'-level in the lake water, and a broad O2 respiration zone of ca 6 mm, we were able to measure H2S depth profiles in the sediment at a good resolution, that allowed for calculation of specific sulfate reduction and H2S oxidation activities. Such calculations showed highest sulfate reduction activity in the anoxic sediment down to ca 20 mm depth A comparison of calculated area1 rates of O2 consumption and sulfate reduction Indicated that sulfate reduction accounted for up to 13 % of total organic carbon mineralization in the acidic sediment. All produced H2S was reoxldized aerobically with O2 at the oxic-anoxic interface. In addition to its good performance in acidic environments, the new H2S microsensor has proven useful for sulfide measurements in neutral and moderate alkaline (pH < 9) biofilms and sediments, and thus is a true alternative to the traditionally used potentiometric Ag/Ag2S microelectrode for most applications in aquatic ecology and biogeochemistry.