Amorphous hard carbon films of varying sp(2)/sp(3) fractions were deposited on Si using filtered cathodic are deposition with pulsed biasing. The films were heat treated in air at temperatures up to 550 degrees C. Raman investigation and nanoindentation were performed to study the modification of the films caused by the heat treatment. It was found that films containing a high sp(3) fraction sustain their hardness for temperatures at least up to 400 degrees C, their structure for temperatures up to 500 degrees C, and show a low rate of material loss during heat treatment. Films containing a low sp(3) fraction graphitize during heat treatment, show changes in structure and hardness, and have a high rate of material loss.