The ennoblement of the free corrosion potential (E-corr) of AISI 316L stainless steel which did not occur in synthetic fresh water (SFW), was observed after introduction of glucose oxidase (Gox) and glucose, or of hydrogen peroxide (H2O2). The composition of the surface was monitored using AFM and XPS, a detailed XPS analysis being based on the discrimination between oxygen of organic and inorganic nature proposed in a previous study. In H2O2 medium, the main changes regarding the inorganic phase were the increase of the oxygen concentration in the passive film, the increase of the molar concentration ratio of oxidized species Fe-ox/Cr-ox and the growth of nanoparticles, presumably made of ferric oxide/hydroxide. In Gox medium, no significant changes were observed in both oxygen concentration and Fe-ox/Cr-ox ratio, but the density of colloidal particles decreased, indicating a dissolution of Fe oxide/hydroxide under the influence of gluconate. In contrast with H2O2, in SFW and Gox the amount of organic compounds increased due to the accumulation of polysaccharides and proteins. The influence of glucose oxidase on the ennoblement of stainless steel is not due to indirect effects of H2O2 through the change of surface composition. The E-corr ennoblement seems to be directly due to the presence of H2O2 and to the electrochemical behavior of H2O2 and related oxygen species. This consideration is important for understanding and controlling microbial influenced corrosion. (C) 2008 Elsevier Inc. All rights reserved.