Three Stainless steel grades S30400, S31600 and S31803 were field tested in the final stages of wastewater treatment plants using anaerobic and aerobic purification processes. Open circuit potentials (OCP) for stainless steels were measured both in aerobic and anaerobic treated wastewater. No ennoblement occurred in the anaerobic purification, but a decrease in the OCP to as low values as -500 mV/SCE. However, in the aerobic purification ennoblement occurred to as high values as +350 mV/SCE after 13 days exposure, but the potential decreased after 50 days to low values, -500 mV/SCE, similar to values obtained in anaerobic purification processes. The decrease in OCP was attributed to the sludge growth surrounding the exposed coupons. Treated wastewater was collected from an aerobic purification stage with high oxygen demand also causing a decrease in the OCP. Investigations were undertaken to reproduce ennoblement under these circumstances. Ennoblement was obtained in the laboratory only after oxygen and a catalytic acid were supplied to the water. Polarization measurements were also undertaken for exposed stainless steel surfaces in anaerobic treatment and compared with surfaces in aerobic treatment. Furthermore, a case was discussed where extensive corrosion occurred on S31600 in anaerobic sludge under oxidative conditions.