The objective of this paper was to investigate how the use of the pulsed mode affects the H atom and CH3 radical densities in H-2/CH4 discharges obtained using a microwave cavity coupling system. The species densities as well as gas temperature and electron energy distribution functions were obtained using a quasi-homogeneous discharge model. The model input parameters, such as the variation of input power and the plasma volume, were determined experimentally. Experimental measurements were also used to determine the gas temperature from the Doppler broadening of the Balmer Halpha line variation, for model validation purposes. The experimental time variations of the gas temperature are characterized by a sharp increase at the beginning of the pulse (t<250 μs) and a decrease to a stable value at the steady state (t>1 ms). The in-pulse steady-state temperature obtained from the model is in agreement with the measured one, although a discrepancy appears in the shape of the early time variation. Modeling results showed that for a given input time-averaged power, the H atom and CH3 radical densities were substantially enhanced by using a small duty cycle (<50%). (C) 2003 The Electrochemical Society.