The bacterial reduction of sulphate is accompanied by oxygen isotope exchange reactions. The mechanisms were not investigated in detail but it is suggested that the formation of sulphate-enzyme complexes as intermediary reaction products are facilitating the exchange of 18O between water and sulphate. Under the experimental conditions the observed fractionation factor is close to 25‰ at 30°C and approaches 27‰ at 17°C. Extrapolated values from a field study indicate a difference of ∼ 29‰ at ∼ 5°C.