Different bi-metallic palladium-copper catalysts have been prepared according to different sequences of impregnation of a commercial alumina. CO adsorption followed by FTIR spectroscopy was performed in order to identify the nature of the superficial sites. Two test reactions were also used to characterize the nature of the active surface site : gas phase toluene hydrogenation was selected in order to show the hydrogenating activity of metallic palladium, whereas the reaction of decomposition of ethanol, which exhibits 100% selectivity in the formation of ethanal on pure copper catalysts, was also studied on this series of catalysts. The results obtained by both methods are complementary. By comparison with the behaviour of pure palladium or copper based catalysts, they allow to propose hypotheses on the surface composition : separated phases, alloys or a phase covered by the other one.