Films were produced from trifluorotoluene/hydrogen (TFT/H-2) and trifluorotoluene/tetrafluoromethane (TFT/CF4) mixtures by plasma-enhanced chemical vapour deposition (PECVD) in a vacuum system fed with RF power. Actinometric optical emission spectroscopy was used to determine the trends in the plasma concentration [H] of the species H as a function of the proportion of the comonomer (H-2 or CF4) in the plasma feed. Also, the plasma concentration of the species CF2, [CF2], was measured as a function of the proportion of CF4 in the feed. The observed rise in [H] with increasing H-2 flows, and a rise in [H] with increasing CF4 flows, are attributed to the increased supply of H and CF2 units, respectively, to the discharge. On the other hand, an observed rise in [H] with increasing CF4 flows implies increased gasphase reactions involving the comonomer or plasma/polymer-surface reactions. X-ray photoelectron spectroscopy (XPS) of the films deposited at various proportions of H-2 and CF4 indicates the presence of CHa, CH-CF, C=O, CF and CF2 groups. The degree of fluorination of the films decreases as the proportion of H-2 in the feed is increased and rises with increasing proportions of CF4.