The susceptibility of Inconel 600 to stress corrosion cracking (SCC) in the primary water of a pressurized water reactor (PWR) is strongly dependent on potential. In the present paper we assess the validity of the hypothesis that the potential dependency of SCC is related to the influence of potential on the identities of the surface films that form on Inconel 600. That is, according to the hypothesis, SCC requires the presence of a particular surface film. The identities of the surface films that develop at different potentials on Inconel 600 in PWR primary water (2 ppm LiOH + 1200 ppm H3BO3) at 288°C were investigated in situ by surface enhanced Raman spectroscopy (SERS). To help identify the components of the films that grow on Inconel 600, the films that form on unalloyed nickel, chromium, and iron in 288°C PWR primary water were also investigated. The main results of the in situ SERS investigation of the surface films are as follows. (1) No films were formed on Inconel 600 at potentials below the region of potential in which SCC occurs. (2) A chromium-rich M3O4 oxide forms on Inconel 600 in the SCC region. (3) NiO forms as the potential is increased immediately above the region of SCC susceptibility. (4) At still higher potentials, films of (Fe,Cr)2O3 and Ni3-xFexO4 form. The results are consistent with concept that specific films affect SCC susceptibility.