The thermal stability of the microstructure and surface mechanical properties of two types of hydrogenated carbon film ("hard" and "soft" versions, both about 2000 Å thick) deposited from methane onto Si(100) wafers by a r.f.-plasma-assisted chemical vapour deposition process have been investigated. Whilst Raman spectroscopy indicates the presence of some degree of sp3 bonding in the hard film, the soft coating luminesced and burned away easily in the laser beam. IR reflectance shows the presence of CHx in both films but the amount, and whether it is CH2 or CH3, could not be deduced owing to the strong IR absorbance of the silicon substrate. On annealing in air (in the temperature range 20-600 °C), scanning electron microscopy and energy-dispersive X-ray analysis show that both films are completely oxidized by 500 °C. On annealing in vacuum (at 10-6 Torr or less) a system of bubbles forms in the hard film at about 530 °C, owing to the liberation of trapped argon, and leads to decohesion of the coating-substrate interface. Also, after annealing in vacuum, transmission electron microscopy shows further marked changes in the nanostructure of the films at elevated temperatures. As expected from these microstructural changes, the microhardness and contact damage resistance of both films are drastically degraded at only relatively modest temperatures. Further, the hard composite exhibits thermal hysteresis of hardness, suggesting the presence of significant compressive residual stress in the film. These results are presented and discussed alongside further insights gained from surface displacement experiments with a nanoindenter.