A thin wafer of polycrystalline Si was mounted on the cathode of a small cylindrical plasma cell which consisted of two flat stainless steel electrodes and a piece of alumina tubing. A discharge was supported by a slow flow of Ar, and short pulses of SF6and O2were injected simultaneously into the Ar stream. The contents of the plasma cell were sampled continuously, and analyzed by mass spectrometer. The concentration transients of various species resulting from the SF6and O2pulses were measured as a function of time. The maxima of the SiF+3transients and thus the etching rates were found to depend on oxygen concentrations in the mixtures, but far less so than the atomic fluorine concentrations, as reported by others. By contrast, the maxima of several S–F and S–O–F species concentrations depended very strongly on O2concentrations. It is concluded that neither atomic fluorine, nor any one of the other observed species can be the sole important reactive species in the etching reaction studied. The experimental results also bear on the fate of SiF2initially formed and thus illustrate the potential of the pulse injection method used in mechanistic studies.