The efficiency of B and As in gas-phase doping of hydrogenated amorphous silicon (a-Si: H) prepared by r.f. sputtering has been experimentally evaluated. The dopant incorporation, i.e. the solid-phase concentration, was found to be linear with gas-phase concentration for both dopants, and independent of the substrate temperature. The creation of defects with doping has been investigated by electron spin resonance and photothermal deflection spectroscopy. The results suggest that a dangling bond is created for each fourfold-coordinated dopant atom, in agreement with the general concept of the modified 8 −Nrule. However, the variations of the doping efficiency with the solid-phase concentrations of B and As (Csol) are ν ∼C0soland ν ∼C−2/3solrespectively, instead of the predictedC−1/2soldependence. We conclude that the laws of mass action cannot be applied under conditions of sputtering and propose that the incorporation of dopant atoms occurs in a non-equilibrium process at the growing surface, in contrast to previous findings on glow-discharge a-Si:H.