An original method is developed to investigate vibrational properties of thin films using infrared (IR) ellipsometry. The procedure is based on in-situ measurements as a function of deposition time. Out of absorption bands, the IR refractive index can be determined and related both to film density and oscillator strengths of electronic transitions. Thanks to band decomposition performed for a given film at different thicknesses, line frequency, band width and intensity of each vibration can be precisely determined, even in case of band overlapping or low IR oscillator strengths. As an illustration, C-H bonding of 1000 Å-thick plasmadeposited polymerlike hydrogenated carbon (PLC) films has been studied. Analysis of the interface between highly saturated PLC films and crystalline silicon reveals the formation of a 20 Å interlayer. Bulk analysis evidences weak vibrations located at 1405 and 1440 cm−1 which had not been observed so far and which are related to π bonds. By combining elastic recoil detection measurements and IR analysis, effective charges of sp3 CHn groups have been calculated. For methyl and methylene groups, these charges are similar to those of hydrocarbon compounds. As far as the sp3 CHn group is concerned, a strong decrease of the effective charge versus film density is evidenced and attributed to the formation of distorted weakly hydrogenated regions induced by ion bombardment.