It has been postulated that bacterial sulphate reduction is related to pH-values above 5.5 (e.g. ). First investigations for the 80-year old dump Plessa in the Lower Lusatian mining district (East Germany) show evidence for active sulphate reduction. Mining dump waters from the Plessa site have pH values of 2.4 to 4.9 and pe values between 2 and 6. Measured sulphide concentrations range from 50 to 1200 -g/L. The influence of bacterial sulphate reduction on the development of sulphate concentrations in the dump waters is supported by increasing δ34S and δ18O values with decreasing sulphate concentrations. These findings document that sulphate reduction under natural conditions is not exclusively related to neutral pH values and strongly reducing conditions. Furthermore the tertiary organic matter contained in the dump sediments is the main carbon source for sulphate reducing bacteria.