A hydrogenated amorphous silicon-carbon (a-Sb0.76C0.24:H) film has been prepared via the glow discharge decomposition of SiH4 and C2H4. The optical constants of this alloy film have been determined as a function of annealing temperature T-a for photon energies between 1.5 and 4.75 eV. The refractive index n and its imaginary part k show small but significant variation with annealing temperature. The optical energy gap E(opt) also exhibits interesting variation with annealing temperature, decreasing with increasing annealing temperature up to T-a = 650 degrees C and then increasing above this temperature. Further, E(opt) is found to be correlated with the inverse of the full width at half-maximum of the Si-C and the Si-O IR stretch absorption mode, which seems to indicate that the changes in E(opt) are structural in origin and that phonon order correlates well with electronic order.