A comparative study of two series of a-C:H films prepared by different techniques and presenting strongly contrasting properties was carried out using several complementary techniques probing the electronic density of states and the incorporation of hydrogen. The results were analysed using a two-phase model consisting of pi-bonded clusters embedded in an amorphous sp3 C:H matrix. It is shown that the two series differ essentially in the sizes of the clusters and in the degree of confinement of the pi electrons within the clusters, and that hydrogen plays a significant role in the film microstructure and its modifications upon annealing.