We developed a microsensor for the amperometric detection of dissolved hydrogen sulfide, H₂S, in sediments and biofilms. The microsensor exhibits a fast (t₉₀ <0.2 to 0.5 s) and linear response to H₂S over a concentration range of 1 to >1000 μmol H₂S l^-1, and has a low stirring dependency of the microsensor signal (<1 to 2%). We used the new microsensor 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 SO₄ ^2- 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 areal 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 reoxidizecl 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.