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 &sgr;nand to derive the nonlinear coefficientsn2and &khgr;(3); these suggest useful strong optical nonlinearities in these amorphous alloys.