A first derivative CKLLAuger electron spectrum from an Fe film which was heated to react with adventitious surface hydrocarbons is presented as a “finger print” for the identification of an iron–carbon reaction. The iron carbide peak shape is significantly different from that of the adventitious hydrocarbons prior to heating the sample and a subsequent 1 min argon ion sputter at 1 keV or that of bulk graphite [K. M. Geib, C. W. Wilmsen, J. E. Mahan, and M. C. Bost, J. Appl. Phys.61, 5299 (1987)]. The carbon is probably bonded as Fe3C because this compound is thermodynamically more stable than the other iron carbides and because the carbon is in an iron rich environment.