The role of ATP-sulphurylases in the enzymology of accelerated primary sludge solubilisation utilising sulphate reducing systems was investigated. ATP-sulphurylase levels were monitored in methanogenic (ATPS(MR)) and sulphidogenic (ATPS(SR)) bioreactor systems. ATPS(MR) and ATPS(SR) activities were mainly present in the cell free fractions of the bioreactor sludge. ATPS(MR) activity was significantly lower (10-20-fold) than the activity observed for ATPS(SR). Studies of pH optimisation showed a pH optima of 9.0 for ATPS(SR). ATPS(MR) and ATPS(SR) exhibited temperature optima of 50 and 55degreesC respectively, with ATPS(SR) showing a higher degree of temperature stability at its temperature optimum. Sulphate and sulphide had no effect on ATPS activity. The effect of volatile acids, lime (CaO) and divalent metal ions on ATPS activity was also investigated. Mg2+ ions were required for activity. ATPS(SR) activity rapidly increased in the closed system sulphidogenic reactor over the first 3-5 days, coincident with sulphate concentration decreases. In conclusion, ATPS activity therefore appears to be a possible marker of sulphate reduction and anaerobic digestion of sludge using a sulphidogenic bioreactor system. Much more research, however, needs to be performed in order to verify a definite and quantitative relationship between ATPS activity and the presence of an active SRB population.