A molecular beam of Cl2 is reactively scattered from a single crystal GaAs (100) surface. Analysis of the mass spectra and of the angular and velocity distributions for the volatile reaction products over the surface temperature range 330–950 K shows that the major volatile reaction products are AsCl3, As4, As2, GaCl3, and GaCl. To correctly identify the GaCl3 and AsCl3 reaction products by mass spectrometry, the vibrational energy dependence of the daughter ion fragmentation patterns was measured by injecting GaCl3 and AsCl3 from a heated nozzle into the mass spectrometer. To convert the ion signals from the mass spectrometer (which are proportional to product density in the ionizer region) to quantities proportional to product flux from the surface, measurements of the product angular and velocity distributions were utilized. The relative product fluxes were then determined by assuming stoichiometric etching of GaAs. The five relative sensitivity factors determined from this fit are consistent with stoichiometric etching and achieve a chlorine mass balance to within 5% over the entire temperature range studied (330–950 K). The absolute reaction probability was determined by comparing the product fluxes and the flux of unreacted Cl2, and the absolute etch rate was established by correlating the ion intensities measured for the evaporation of GaAs with the absolute evaporation rate reported in the literature. For a Cl2 flux of 1.75×1015 molecules/cm2/s and a surface temperature of 750 K, the reaction probability is 0.42±0.05 and the etch rate is ∼2 ML/s. Finally, the effect of translational energy on the absolute etch rate and relative product flux distributions was measured by changing the rare gas diluent of the adiabatic expansion. No change in the etch rate or relative product yields wa- - s observed within experimental error for a variation of 0.025–0.15 eV in the translational energy directed along the surface normal.