Plasma hydrogenation at temperatures between 50° and 700°C of thermally grown films on Si has been investigated by infrared absorption and nuclear reaction analysis techniques. Hydrogen profiles after hydrogenation are consistent with trap‐limited diffusion with a diffusivity of 10−13 cm2/s a 100°C. Hydrogen bonds predominantly to O in the host. Isotope exchange occurs on these OH bonds in a deuterium plasma, and the bonds are unstable at temperatures >500°C. Predominance of OH over bonds in thermal oxide films is in contrast to significant bond formation found by Shelby for hydrogen‐impregnated vitreous silica damaged by gamma‐ray irradiation. Bond selection by H in displacement‐damaged oxides may, therefore, be an indicator of deviation from stoichiometry.