The oxidation kinetics of a-CHx overcoats during exposure to oxygen and water vapor have been measured using X-ray photoemission spectroscopy (XPS) in an apparatus that allows oxidation and analysis of freshly deposited a-CHx overcoats without prior exposure of the overcoats to air. The uptake of oxygen on the surfaces of the a-CHx overcoats has been measured at O-2 and H2O pressures in the range 10(-7)-10(-3) Torr at room temperature. The uptake of oxygen during O-2 exposures on the order of 10(7) Langmuirs leads to saturation of the a-CHx overcoat surfaces at oxidation levels on the order of 20%. This indicates that the surfaces of a-CHx overcoats are relatively inert to oxidation in the sense that the dissociative sticking coefficient of O-2 is similar to 10(-6). Oxygen uptake during exposure to H2O vapor is similar to the uptake during exposure to O-2 gas. Although the surfaces of the a-CHx overcoats are quite inhomogeneous, it has been possible to model the uptake of oxygen on their surfaces using a fairly simple Langmuir-Hinshelwood mechanism. Interestingly, the saturation coverage of oxygen during exposure to air at atmospheric pressure is similar to 6%, significantly lower than that obtained during low-pressure exposure to O-2 gas or H2O vapor.