We have studied nonlinear refraction and absorption of band-gap tuned hydrogenated amorphous silicon-carbon alloys using 532-nm picosecond degenerate four-wave-mixing, transient grating and excite/probe techniques. The highest induced absorption cross sections are obtained for low band-gap samples. By comparison, grating diffraction efficiencies increase with band gap, indicating stronger refractive index contributions. The results from the four-wave-mixing and excite/probe experiments are combined to calculate refractive index cross sections sn and to derive the nonlinear coefficients n 2 and ?(3); these suggest useful strong optical nonlinearities in these amorphous alloys.