The surface chemistry of coadsorbed CO and metal adatoms on the Pt(100) surface in acid electrolyte has been studied by means of rotating ring disk electrode (RRDE) measurements. These results showed that Cu and Pb adatoms are readily displaced from the Pt(100) surface by CO. The consequence of the observed displacement phenomenon is that the kinetics of the electrooxidation of CO on Pt(100) surfaces in Cu2+- (Pb2+-) containing solution is the same as in Cu2+- (Pb2+-) free solution.