For the interpretation of experimental findings and the development of concrete mechanisms of the GaAs etching by chlorine, quantum chemical calculations were executed with molecular models of local structures of flat surfaces and steps with various chlorinations. Desorption energies and, partly, desorption barrier heights of Ga-2, GaClm (m = 1-3), AS(2), AS(4), and AsCln (n = 1-3) are calculated by the density functional theory (B3P86). The obtained data agree well with qualitative and quantitative experimental findings and allow to explain the reaction behavior. Chlorine molecules preferably bond to Ga atoms than to As atoms. At steps, Cl atoms can pass over from As to Ga atoms between the first and second layer by overcoming small barriers. In general, the desorption energies of GaCln and AsCln (n = 1-3) species show a characteristic behavior with increasing number of chlorine atoms at neighboring positions of the desorbing species. The reaction product distribution can be interpreted in dependence upon the temperature and the Cl-2 concentration at the surface: Under deficient chlorine concentration (high temperatures), the GaCl and As-2 desorption and, under high chlorine concentration (low temperatures), the GaCl3 and AsCl3 desorption is preferred. In agreement with experiments, the desorption energies of the essential reaction products increase in the order: AsCl3, GaCl3, GaCl, and As-2. GaCl2, AsCl2, AsCl are relatively stably bound at the surface.