Sewage sludge contains a significant concentration of sulphur (0.3-2.3 wt%) that hampers sludge (co-)combustion as the final disposal route due to the formation and emission of gaseous SO2 and associated acid rain problems. It is therefore of paramount importance to limit the sulphur content of waste-activated sludge (WAS) as much as possible. This paper studies the distribution and transformations of sulphur compounds occurring during the successive steps in the sewage sludge treatment. Data were gathered from sampling and analyzing all relevant water and sludge streams of a full-scale treatment plant during I month. It was seen that sulphates are the predominant compounds in secondary sludge. During thickening, the oxygen level in the sludge decreases due to microbial activity, and the sulphates are gradually transformed into sulphides. The process continues when thickened sludge is stored in the sludge storage tanks. After anaerobic digestion, the ORP of the sludge has decreased to such an extent that all inorganic sulphur is transformed into sulphides. The experimental results were confirmed by a chemical speciation. To limit the residual sulphur content in the sludge, it is hence beneficial to dewater the sludge when still in aerobic conditions, thus largely releasing soluble sulphates with the sludge water.