X-ray photoelectron spectroscopy has been used to monitor the composition of the surface film formed when a clean Mg metal is subjected to pretreatments that simulate exposure to ambient environments. The results indicate that an as-received (commercial) Mg metal contains a surface covered by a film constituted predominantly by magnesium hydroxide and a smaller but appreciable quantity of magnesium bicarbonate. These observations have important ramifications in the mechanistic description of the Grignard reaction since the interaction between Mg metal and an alkyl halide must contend with the surface hydroxide and bicarbonate films.