The influence of the presence of H 2 on the physico-chemical properties of a Pd(10wt.%)/γ-Al 2 O 3 catalyst was investigated. Catalysts were treated under methane combustion conditions with various amounts of H 2 in the feed. Samples obtained were characterized by BET, CO-chemisorption, XRD, XPS and/or ToF-SIMS techniques. The effect of H 2 is essentially to sinter the palladium phase and to help remove surface chlorine coming from the palladium precursor. In presence of low amounts of H 2 , O migration phenomena are suggested to occur to maintain the surface of the palladium particles in an oxidized state while, as the H 2 concentration increases, metallic palladium progressively forms in the bulk. Above a threshold value, the H 2 concentration becomes too high, metallic palladium is formed on the surface.