This paper reports a systematic study of the effect of CO gas on the chemical composition at the surface of gold-based alloys. Using DFT periodic calculations in presence of adsorbed CO the segregation behavior of group 9-10-11 transition metals (Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co) substituted in semi-infinite gold surfaces is investigated. Although, CO is found to be more strongly adsorbed on (100) than on the (111) surface, the segregation of M impurities is found to be more pronounced on the (111) surface. The results reveal two competitive effects: the effect of M on CO and the effect of CO on M. Thus, on one hand, if M exists on the (100) gold facet, CO would be strongly adsorbed on it. But if M is initially located in the bulk, it would segregate to the (111) facet instead of the (100) in order to bind to CO.