Microbially influenced corrosion by manganese oxidizing microorganisms (MIC by MONOs) is a corrosion phenomenon occurring in freshwater systems affecting stainless steels. Typically, chloride induced corrosion is observed at chloride concentrations normally not considered as critical. The initiation of corrosion is attributed to an anodic shift of the open circuit potential of the passive stainless steel caused by manganese dioxide biomineralized by the specific organisms. However, not only the potential of the passive steel seems of interest. For the stabilization of localized corrosion, the ability of the cathodic reaction to drive enough current for promoting the switch from metastable pitting to stable pit growth is considered important. Consequently, experiments have been carried out in order to compare the dynamic cathodic properties of the usual oxygen reduction reaction with that of biofilm incorporating biomineralized MnO2. The results indicate a significantly better cathodic performance of such biofilm which might explain the fast and extensive damages observed in certain cases of MIC by MOMOs. (c) 2006 Elsevier Ltd. All rights reserved.