Microsensor techniques were used to investigate in situ the simultaneous occurrence of sulfate reduction and nitrogen removal in a membrane aerated biofilm reactor. H2S, O2, pH, ORP, NH4+ and NO3 microsensors were fabricated and used to measure the profiles inside the membrane aerated biofilm. Production and consumption rates of H2S, O2, NH4+ and NO3 were estimated using corresponding concentration profiles. The results showed that in anoxic zone, located from the interface between biofilm and bulk liquid to about 550 μm below the interface, both sulfate reduction and denitrification occurred. Highest H2S production rates (around 0.27 mg L−1s−1) were found about 400 to 450 μm below the interface. Below the anoxic zone, an aerobic zone was present. High H2S oxidation activity occurred at around 550–700 μm below the interface. High oxygen consumption rates (0.34 mg L−1s−1) occurred at around 750–900 μm below the interface. Nitrification activity occurred at about 500–650 μm below the interface. Along the entire biofilm depth, pH changed slightly (within 0.2 unit). Near the interface of the aerobic and anoxic zone, there was a drastic redox potential change. These results demonstrated simultaneous sulfate reduction and nitrogen removal in a piece of membrane aerated biofilm.