The surface reaction probability β in a remote Ar–H2–SiH4 plasma used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H) has been investigated by a technique proposed by D. A. Doughty etal [J. Appl. Phys. 67, 6220 (1990)]. Reactive species from the plasma are trapped in a well, created by two substrates with a small slit in the upper substrate. The distribution of amount of film deposited on both substrates yields information on the compound value of the surface reaction probability, which depends on the species entering the well. The surface reaction probability decreases from a value within the range of 0.45–0.50 in a highly dissociated plasma to 0.33±0.05 in a plasma with ∼12% SiH4 depletion. This corresponds to a shift from a plasma with a significant production of silane radicals with a high (surface) reactivity (SiHx,x≪3) to a plasma where SiH3 is dominant. This has also been corroborated by Monte Carlo simulations. The decrease in surface reaction probability is in line with an improving a-Si:H film quality. Furthermore, the influence of the substrate temperature has been investigated. © 2000 American Institute of Physics.