In order to obtain more information about the nature of the hydrogen bonding and the thermal stability of a-C:H films, we have studied two different types of films labeled I and II deposited in a d.c. multipolar plasma system from pure methane at substrate bias equal to -40 and -600 V respectively. A combination of several complementary techniques has been carried out on the samples in their as-deposited state as well as after isochronal annealing cycles at increasing temperatures up to 650 ○C. The results clearly indicate that the two types of samples have very different H bonding and microstructure in their as-deposited state. They also behave differently upon annealing and still exhibit quite different microstructures and H bonding configurations up to high annealing temperature.