We have studied the interactions of atomic deuterium with hydrogenated amorphous silicon (a-Si:H) surfaces using surface-sensitive infrared spectroscopy. We deconvoluted the effects of the abstraction reaction from insertion and etching reactions and determined the activation energy barrier for abstraction of H by D from a-Si:H surfaces. Both abstraction and insertion are observed in our experiments conducted over times ranging from a few seconds to hundreds of seconds and sequential insertion of D eventually results in the formation of deuterated silane and etching of the film. The abstraction rate is found to be independent of temperature with an essentially zero activation energy barrier (0.011+/-0.013 eV), consistent with an Eley-Rideal mechanism and in agreement with recent atomistic calculations.