An impedance model based on three state-variables (Pt-CO, Pt-H, and Pt-OH) is proposed for the kinetics investigation of H-2/CO electro-oxidation on Pt/C and its alloys. The simulation results of Pt/C in a high potential range exhibit unusual reversal behavior of the impedance pattern in the II and III quadrants. This behavior reveals the change of the rate-determining step from CO oxidation to CO adsorption. The experimental impedance results of the alloy PtRu/C are in agreement with the simulation studies, and suggest that the enhanced CO oxidation observed on the alloy can be well explained by the promoted OHads, generation on the Ru surface at low potential. Meanwhile, a different reaction mechanism has been elucidated for the PtSn/C, where an improved activity towards CO oxidation is noticed. This can be attributed to the combination of factors such as promoted OH nucleation. exclusion of CO adsorption on the Sn site and the presence of lower CO adsorption coverage on the Pt site. (C) 2003 Elsevier B.V. All rights reserved.