The transient grating technique was used to study the optical properties of hydrogenated amorphous silicon films. A carrier lifetime &tgr;e=3.3 &mgr;s, a diffusion coefficientD=4.4×10−2cm2/s, and effective third order nonlinear susceptibility &khgr;(3)=(5.1±1.0)×10−7esu were measured for &lgr;pump=532 nm. The electronic and thermal contributions of the grating were estimated by diffracting a continuous wave HeNe laser beam. For intensities 1.4–4.1 mW/cm2, ∼68% of the response at 633 nm is of electronic origin. A slowly rising diffraction component was also observed. ©1995 American Institute of Physics.