The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H,SiH3,andSi2H5on a growinga-Si:H film, andCH3andC2H5on ana-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. ForSiH3ona-Si:H,β=0.28±0.03in excellent agreement with previous determinations using other experimental approaches, and forSi2H5,0.1<β<0.3.For H ona-Si:H,0.4<β<1and mostly consists of surface recombination asH2,while the etching probability of Si asSiH4is onlyε≈0.03at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution ofSiH4inH2the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surface recombination at the expense of sticking. ForCH3orC2H5ona-C:H it is shown that β is not constant during the discharge afterglow, decreasing from about 0.01 down to 0.001. This reveals that chemisorption of these radicals on the H-saturateda-C:H surface is entirely governed by the competition between desorption and creation of active sites by ion bombardment or H atoms. The differences between the surface reaction kinetics ofSiH3ona-Si:H andCH3ona-C:H are discussed within a unified model of precursor-mediated chemisorption.