Diamond-like carbon (DLC) thin films were deposited on stainless steel and silicon substrates by a r.f. (13.56 MHz) magnetron sputtering technique. A carbon target (99.99%) and a gas mixture of Ar/CH4 were used. During the deposition process the plasma discharge was monitored by optical emission spectroscopy (OES) in order to analyze the state of the chemical species present in the plasma. The films were characterized by Raman spectroscopy and by reflection, absorption and transmission infrared spectroscopy. The morphology of the deposited layers was analyzed by scanning electron microscopy (SEM). The Raman intensity of the diamond and graphite peaks (I-D/I-G) depends on the percentage of CH4 in the gas mixture. The relationship between the lines H-alpha, and H-beta, intensities is a measure of the relative change of the plasma electronic temperature that, for the experimental conditions, does not depend significantly on the concentration of CH4 in the mixture with a value of the order of 1 eV. Optical emission spectroscopy shows that, besides the atomic hydrogen peak (H-alpha, H-beta, H-gamma), emission spectra are dominated by neutral CH specie and the most intense peak in the spectra correspond to CH (A(2)Delta --> X-2 Pi at 431.5 nm) which is supposed to be the precursor species in the diamond-like films. In the transmission infrared spectroscopy analysis the sp(3) CH2 symmetric and asymmetric at 2870 and 2960 cm(-1) stretching peaks were observed. These peaks have also been observed in diamond deposited at high CH4 concentrations and also in diamond-like carbon (DLC) films.