The oxide films formed on the surfaces of melt-spun Mg exposed to air, immersed in distilled water or 3% NaCl solution saturated with Mg(OH)I have been investigated by X-ray photoelectron spectroscopy (XPS). High resolution XPS spectra revealed two distinct oxygen species on the surface films: one assigned to O2- in MgO, the other to OH- in Mg(OH)(2). Depth profiling revealed that the two species had different depth distributions in the films. The oxide film formed in air comprised a contamination outer layer and a relatively thick (5-6 nm) predominantly MgO inner layer. The film formed in distilled water or 3% NaCl solution saturated with Mg(OH)(2) was mainly a mixture of Mg(OH)(2) and MgO. Mg(OH)(2) was predominant at the top layer and decreased gradually with depth while MgO exhibited the opposite behavior. The corrosion product formed in 3% NaCl solution was more hydrated and much thicker that the films formed in the other two conditions. Cl- ion was incorporated in the oxide film formed in 3% NaCl solution. There exists both partial and complete dissociation of adsorbed water when melt-spun pure Mg ribbons are immersed in distilled water or 3% NaCl solution saturated with Mg(OH)(2).