Hydrogenated amorphous carbon (a-C:H) thin films are deposited at low-pressure (0.35 Pa) on c-Si substrates from methane plasmas. The plasma enhanced chemical vapor deposition (PECVD) process is operated in a dual electron cyclotron resonance (ECR)- radio frequency (r.f.) reactor using an independently controlled r.f. substrate bias. The film,growth and substrate temperature are monitored in situ using real time kinetic ellipsometry and fluoroptometry. The deposited films are investigated by means of ex situ UV-Visible (1.5-5.0 eV) spectroscopic ellipsometry and Fourier transform infrared spectroscopy (500-4000 cm(-1)). The dielectric functions are determined from the ellipsometric spectra using Jellison and Modine's dispersion model whereas the structure of the films is examined through the use of absorption spectra. Complementary Raman measurements performed at 457 nm reveal the presence of the two main graphitic G and D peaks as well as two additional bands located at about 1170 and 1470 cm(-1). It is shown that a-C:H film optical properties change from polymer-like to diamond-like depending on the energy and flux of the hydrocarbon species impinging on the substrate.