Hydrogenated amorphous carbon films (a-C:H) have been prepared by r.f. plasma deposition from a C3H8-Ar gas mixture. Visible, infrared and Raman spectroscopy as well as electrical measurements are reported and used to characterize the bonding and electronic properties of these films. The gas phase discharge has been investigated by Langmuir probe and by optical emission spectroscopy diagnostics. Under constant self-bias voltage and total pressure, variations in the propane partial pressure brought into evidence the influence of gas phase scission of hydrocarbon bonds on the growth process and the layer composition. The change appears with the increase of the: propane partial pressure, which leads to a decrease of both electron temperature and electron density. Simultaneously, CH optical emission increases with respect to the carbon emission. The decrease in the electron temperature results in a decrease of the discharge efficiency to break CW and carbon bonds, and thus, in a decrease of carbon atom production in the gas phase. The formation of more hydrogenated radical precursors, as a consequence of the above, enhances the plasma polymerization process with respect to graphitic nucleation. Films deposited at high electron temperatures were found to contain a large amount of graphite, while those prepared at lower temperatures were found to be polymer-like, with a high hydrogen content.