An Auger in‐depth analysis and a 350 keV He+backscattering technique were used to investigate the growth of an SiO2protective layer and the compositional change in sputtered molybdenum silicide films after oxidation, respectively. Silicon depletion and molybdenum pileup in molybdenum silicide were observed near the SiO2/Mo‐Si interface. The film becomes molybdenum rich with increasing oxidation time. The Si/Mo atomic ratio decreases rapidly in the first 30‐min period and then decreases slowly, nearly proportionally to the square root of the oxidation time. These behaviors are explained by the preferential oxidation of silicon and the reduced silicon present at the SiO2/Mo‐Si interface from the rest of the molybdenum silicide layer.