The dependence of the properties of hydrogenated amorphous silicon-carbon alloys deposited with a 1:10 silane (SiH4) to methane (CH4) or ethylene (C2H4) gas flow ratios with the hydrogen dilution, deposition pressure and power is studied. In methane-based films the carbon content shows a decrease (from ≈0.75 to 0.55) with increasing hydrogen dilution from 0 to 98%, while the ethylene-based films show no dependence of the carbon fraction (≈0.9) on hydrogen dilution. The photoconductivity shows an increase for hydrogen dilutions above 90= for both methane and ethylene-based films. While this increase is attributed, in the case of methane, to the observed decrease in carbon content, no corresponding decrease in carbon content is observed in the ethylene-based films, suggesting a decrease in the density of recombination centers with hydrogen dilution. The Urbach tail and the room-temperature photoluminescence peak correlate with carbon content independent of the carbon source-gas and deposition conditions used.