Molecular chlorine is scattered from a GaAs(100) surface and the reaction products detected by a mass spectrometer. A careful analysis of the velocity and angular distributions of the reaction products as well as the vibrational energy dependence of the daughter ion fragmentation pattern shows that the steady state reaction is stoichiometric in the removal of Ga and As until the incongruent evaporation rate exceeds the etch rate induced by the incident chlorine flux. The depletion of the nonreactive molecular chlorine signal is accounted for by the flux of chlorinated reaction products. The absolute etch rates as well as the incident chlorine flux are determined from the known evaporation rate of GaAs at the incongruent evaporation temperature. Modulated molecular beam scattering is used to establish some necessary reaction steps.