Using x-ray photoelectron spectroscopy (XPS) and real-time, laser-induced thermal desoprtion-laser-induced fluorescence (LD-LIF), we have determined the coverage of Br and Cl on Si(100) surfaces that are etched in mixed HBr/Cl-2 plasmas. Halogen coverages measured by XPS after etching are directly proportional to the fraction of the respective halogen in the feed gas. LD-LIF was detected from SiCl(g) and SiBr(g) products with intensities that are a semiquantitative measure of instantaneous Cl and Br coverages. Saturated coverages during etching in Cl-2 and HBr plasmas are 1.0 X 10(15) Cl/cm(2) and 6.0 X 10(14) Br/cm(2), respectively. Etch rates at these tw extremes are 2170 and 1330 Angstrom/min, and therefore are proportional to the respective halogen coverages. it therefore appears that the rate of formation of volatile Si-halides, stimulated by ion bombardment, is lower for HBr mainly because less halogen is available on the surface at saturated coverage. Langmuir probe measurements indicate that the ion flux is 17% lower in pure HBr plasma, compared with Cl-2 plasmas at the same power. The plasma potential, direct current bias voltage, and hence ion energy were nearly constant, however, over the range of gas mixtures. The slightly different ion fluxes suggest that the ion bombardment-stimulated process, although similar for Cl-2 and HBr plasma, is actually slightly more efficient on a per halogen basis for Br versus Cl. Positive photoresist surface that were also exposed to the plasma differ from Si in that less Br adsorbs on the surface both in pure HBr and mixed HBr/Cl-2 plasmas. Apparently the lower reactivity of photoresist with Br (compared with Cl) is at least partly responsible for the widely observed slower etch rate of photoresist in HBr plasma, and hence enhanced selectivity when etching Si.