Surface enhanced Raman scattering (SERS) was used to observe interactions of dilute Cr-VI solutions with silver and copper surfaces in situ. Using silver as a model surface which supports strong SERS with a 514.5 nm laser, it was possible to observe Cr-III at the near monolayer level, and the spectra were compared to those from Cr-III oxyhydroxide species and Cr-III/Cr-VI mixed oxide. Similar experiments were conducted with Cu surfaces and 785 nm excitation. Upon exposure to Cr-VI solution, the characteristic Cu oxide Raman bands disappeared, and a Cr-III band increased in intensity over a period of; 20 h. The intensity of the Cr-III band on Cu became self-limiting after the formation of several Cr-III monolayers, as supported by chronoamperometry experiments. This Cr-III spectrum was stable after Cr-VI was removed from the solution provided the potential remained negative of -200 mV vs. Ag/AgCl. The results support the conclusion that Cr-VI is reductively adsorbed to Cu at the near neutral pH and open circuit potentials expected for Cu/Al alloys in field applications. The Cr-III film is stable and is a strong inhibitor of electron transfer in general and oxygen reduction in particular. An important mechanistic feature of Cr-III formation is the substitution lability of Cr-VI compared to Cr-III. The Cr-VI-O bond can be broken much more rapidly than the substitution inert Cr-III-O bond, making formation of Cr-III/Cr-VI mixed oxide kinetically favorable. Once reduced to Cr-III, however, the substitution inert oxyhydroxide film is much less labile. An important and central feature of Cr-VI as a corrosion inhibitor is its transformation via reductive adsorption from a mobile, substitution labile Cr-VI form to an insoluble, substitution inert Cr-III oxyhydroxide. Furthermore, Cr-VI reduction is likely to occur at cathodic sites previously responsible for oxygen reduction, which are then permanently blocked by a stable Cr-III film with a thickness of a few monolayers. (C) 2003 The Electrochemical Society.