Surface carbon species are known to play an important role in catalytic reactions, in particular, those associated with the conversion of CO to hydrocarbons. Two distinct surface C species have been noted on single crystals of Ni and Ru used as catalysts for the methanation reaction, and these have been associated through their Auger fingerprints with active ‘‘carbidic’’ and inactive ‘‘graphitic’’ forms. In the present paper, we discuss the results of a more detailed study of the Auger line shapes of various C species on Ni(100), Ni(111), and Rh(111) surfaces which include line shape corrections for loss effects. We compare our results with spectra which act as models of the local C environment. We present data which support a multiply coordinated C adsorption site and demonstrates a facile transformation from carbidic to graphitic carbon forms on the Rh surface under thermal annealing. This transformation is shown to manifest itself in the CO2/H2 reaction by a low temperature deactivation of the catalyst surface.