Hydrogenated amorphous carbon (a-C:H) layers deposited at near room temperature by CH4 r.f. discharge have been studied. The role of plasma chemistry and of fluxes of neutrals and ions is discussed. The layers’ properties have been characterized by means of density measurements, UV-Vis absorption, electron spin resonance and electrical conductivity. Hydrogen incorporation, carbon bonding and network structure have been studied by Fourier transform IR and Raman spectroscopies, secondary ion mass spectrometry and X-ray photoelectron spectroscopy. It is shown that the substrate bias voltage (and then the ions’ average energy) is the most important deposition parameter for the surface morphology and for the film properties.