The correlation between the deposition conditions and the structural and optoelectronic properties of a-SiC:H were studied in order to obtain optimized device-quality films. Two different sets of films were deposited by plasma enhanced chemical vapour deposition, with and without H-2 dilution of the mixture of SiH4 + CH4 respectively. An improvement in the optoelectronic properties was observed : the H-2-diluted films show higher photoconductivity, sharper Urbach energy, reduced sub-band gap absorption and reduced CH(n) and SiH2 bond density with respect to undiluted films. The growth mechanisms were interpreted by means of chemical bonding configurations deduced from IR absorptance measurements.