Electron transfer between the NAD-dependent hydrogenase from Ralstonia eutropha and AISI 316L stainless steel was studied with a view to identifying a possible role of hydrogenase in anaerobic microbially influenced corrosion (mic), as already suspected. A thin spectroelectrochemical cell was designed and qualified, taking great care to obtain a reproducible surface state of the stainless steel grid electrode. Successive electrolyses were performed at potential values in the range from -0.60 to -0.90 V(SCE), with different NAD(+) concentrations, and different hydrogenase activities. Hydrogenase significantly enhanced the charge consumed when NAD(+) was present in solution. In the absence of NAD(+) it exerted, on the contrary, a masking effect, like an inert protein. The charge consumed during each electrolysis was compared with the charge derived from the spectrophotometric monitoring of NADH formation. Depending on the potential value, hydrogenase predominantly catalyzed either the reduction of water to hydrogen, or the reduction of NAD(+) to NADH. The current due to the presence of hydrogenase and NAD(+) remained low according to corrosion moities, but it was demonstrated here that the presence of hydrogenase on the surface of stainless steel induces an effective hydrogenase-catalyzed cathodic reaction that must be taken into account in mic. (C) 2003 Elsevier B.V. All rights reserved.