In our work, photoemission electron microscopy (PEEM) was employed to real-time and in situ study CO adsorption on the O-saturated Ag/Pt(110) composite surface. Coupling occur-red between adjacent surfaces with different activities for CO + O-ad reaction. in which adsorbed CO was directly observed to migrate from strongly bound Pt sites to weakly bound Ag sites in the form of reaction-diffusion wave. Our results imply that surface species may migrate not only from weakly bound sites to strongly bound sites (enthalpy favorable process) but also from strongly bound sites to weakly hound sites (enthalpy unfavorable process) during heterogeneous catalysis reactions.