The structure and the stability of bimetallic particles are controlled by their interaction with surface oxide species on support or by support geometry. Location and size of the bimetallic particles estimated by XRD and XPS, depends on the environment and treatment. Thus, the Pt-Co bimetallic particles entrapped in NaY zeolite cages become unstable upon mild oxidation by O2 and/or reaction with surface protons causing the Co2+ ions formed to migrate irreversibly from the supercages into the sodalite cages as determined by TPR and TPD. Differences in the selectivity pattern observed for the CO hydrogenation over alumina and NaY zeolite supported Pt-Co catalysts, can be interpreted by the interaction between bimetallic particles with surface cobalt oxide species and the zeolite cage wall, respectively.