Lakes affected by acid mine drainage (AMD) or acid rain often contain elevated concentrations of ammonium, which threatens water quality. It is commonly assumed that this is due to the inhibition of microbial nitrification in acidic water, but nitrification was never directly measured in mine pit lakes. For the first time, we measured nitrification by 15NH 4Cl isotope tracer addition in acidic as well as neutral mine pit lakes in Spain and Germany. Nitrification activity was only detected in neutral lakes. In acidic lakes no conversion of 15NH4+ to 15NO3- was observed. This was true both for the water column as well as for biofilms on the surface of macrophytes or dead wood and the oxic surface layer of the sediment. Stable isotope analysis of nitrate showed 18O values typical for nitrification only in neutral lakes. In a comparison of NH4+ concentrations in 297 surface waters with different pH, ammonium concentrations higher 10 mg NH 4-N L-1 were only observed in lakes below pH 3. On the basis of the results from stable isotope investigations and the examination of a metadata set we conclude that the lower limit for nitrification in lakes is around pH 3.