The optoelectronic properties of hydrogenated amorphous silicon-carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using radio-frequency glow discharge and optimization was made with respect to deposition power and pressure, hydrogen dilution, and methane (or ethylene) -to-silane gas phase ratio. Regardless of the deposition conditions and source gases used, the optical, structural, and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The Urbach tail energy Eu and the B factor of the Tauc plot correlate with E04 (defined as the energy at which the absorption coefficient is equal to 104 cm-1) taken from photothermal deflection spectroscopy measurements. Up to E04pds≊2.6 eV, Eu increases monotonically from 50 up to ≊200 meV, while the B factor decreases from ≊800 down to ≊200 cm-1/2 eV-1/2. Above E04pds≊2.6 eV, both Eu and B remain approximately constant. The photoconductivity decreases exponentially with E04pds and is below 10-10 Ω-1 cm-1 for E 04pds≥2.6 eV. Room-temperature photoluminescence is observed when E04pds≥2.6 eV. The photoluminescence peak position lies an average of 0.6 eV below the value of E04pds and increases linearly with decreasing value of the B factor of the Tauc plot. © 1995 American Institute of Physics.