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The importance of biochemical conversions of ammonium sulphate in water acidification was studied by means of sediment-water columns at the laboratory and enclosure experiments in natural waters. Evidence is given that acidification induced by ammonium sulphate is determined by ammonium, nitrate and sulphate related processes. The importance of various environmental factors influencing the rate and extent of these processes is experimentally supported. In sediment-water columns with circumneutral water poor in bicarbonate, above an oxidizing sandy sediment, the production of hydrogen-ions was fully explained by the nitrification process. In acid water with pH ≤ 4.5, the rate of nitrification was reduced down to 14 %. The acidifying impact of ammonium sulphate decreased in the presence of nitrogen assimilating algae and reducing sedimentary conditions. In a typical low-alkaline shallow water with an oxidizing sediment the nitrification process contributed significantly to the acidification by ammonium sulphate. In an acid water with a reducing sediment denitrification was the most important alkalinizing process. The acidification efficiency of ammonium sulphate conversions was 132 % in the first, and 22 % in the second type of water. It is emphasized that the effective acid production resulting from high external inputs of ammonium sulphate, e.g. by atmospheric deposition, exerts a significant pressure on the acid-base balance of low-alkaline waters.