Electrochemical techniques and variable angle x-ray photoelectron spectroscopy (XPS) were used to study the passivation of Mo, and plasma-sprayed and hot-pressed MoSi2 in a pitting environment (4M HCl). A thin (12 Angstrom), insoluble Mo oxychloride salt layer formed on the pure Mo electrode following 1 h of passivation at 0 mV vs. saturated calomel electrode. Both MoSi2 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 Angstrom thick SiO2 film incorporating hexavalent Mo for an overall stoichiometry of Mo0.05SiO2. The film formed on the hot-pressed disilicide had primarily the same stoichiometry with slight variation due to the presence of pores.