Electrochemical techniques and variable angle x-ray photoelectron spectroscopy (XPS) were used to study the passivation of Mo, and plasma-sprayed and hot-pressed MoSi 2 in a pitting environment (4M HCl). A thin (12 A), insoluble Mo oxychloride salt layer formed on the pure Mo electrode following 1 h of passivation at 0 mV vs. saturated calomel electrode. Both MoSi 2 samples in 4M HCl displayed a significantly higher passive current density as well as a much higher breakdown potential, indicating a thicker but perhaps less dense passive layer. XPS of the film formed on the thermally sprayed sample revealed a 34 A thick SiO 2 film incorporating hexavalent Mo for an overall stoichiometry of Mo 0.05 SiO 2 . The film formed on the hot-pressed disilicide had primarily the same stoichiometry with slight variation due to the presence of pores