Abstract In-situ spectroscopic ellipsometry (SE) was used to study the initial stages of growth and to determine the tetrahedral (sp 3 ) CC bonds, the thickness, and the deposition rate of amorphous carbon (a-C) thin films, deposited by RF magnetron sputtering. The only deposition parameter varied was the bias voltage ( V b ) applied onto the substrate from +10 to −140 V. The a-C films were deposited in sequential thin layers and in-situ SE spectra, in the energy region 1.5–5.5 eV, were obtained after the deposition of each layer. The measured SE spectra were analysed by means of the Bruggeman effective medium theory. It was found that V b is responsible for the development of a-C films of different composition and properties. Differences in deposition rate and sp 3 content were found from the initial stages of growth not only between films deposited with negative and positive V b , but also between films deposited with (e.g. low, around −5 V and high below −140 V) negative V b . The latter films developed with V b between −40 and −80 V were found to be more rich in sp 3 sites than those developed with V b above −40 or below −80 V. Finally, the deposition rate was found to be almost linear for V b >0 and power-law-dependent, for V b