Secondary ion mass spectroscopy (SIMS) has been used to measure the diffusion of deuterium in hydrogenated amorphous silicon. For a film deposited in a dc glow discharge in SiH4at a substrate temperature of 315 °C, the diffusion data fitsD(T) =1.17×10−2 exp(−1.53 eV/kT) cm2/s. This result implies that degradation of these films due to hydrogen out‐diffusion at 100 °C will not be significant until after more than 104years.