Hard, relatively transparent, amorphous carbon films have potential applications as wear-resistant coatings, including those in the optical industry. We have investigated some of the effects of elevated temperature on these films, deposited by glow discharge from methane. Differential scanning calorimetry (DSC) has shown a broad, asymmetric, exothermic peak at about 550 °C and a larger energy release at about 750 °C. The latter corresponds to a structural transformation (graphitization) and the sample becomes soft and black. The former appears to correspond to the release of hydrogen, similar to Si:H films. An analysis shows that the energy released, as measured in the DSC, is consistent with literature values for the energy of chemisorbed H on carbon and for the amount of ‘‘unbonded’’ H observed from a-carbon films. Models are presented for the film structures needed to account for this chemisorbed H and for the film densities and transmission electron microscope images.