Reaction of single‐crystal GaAs with molecular chlorine was studied with and without simultaneous bombardment by energetic argon ions. The reaction was detected by aninsitumass spectrometer; reflected chlorine and reaction product signals were measured as functions of crystal temperature. A modulated molecular beam of intensity 1×1017cm−2 s−1was used for the purely thermal reaction tests. At temperatures greater than 550 K, the monochlorides of Ga and As were observed in equal quantities. At lower temperatures, only the trichlorides were observed. A maximum reaction probability for trichloride production of 0.3 was observed at 450 K. The effect of modulated ion bombardment with a steady Cl2beam was also examined. The shapes of the waveforms of the desorbed products observed in this experiment suggested that the gallium‐chloride‐producing reaction was stimulated by ion bombardment. In the absence of ion‐assisted desorption, the gallium chloride surface builds up to a saturation coverage of ∼1015Ga atoms/cm2. This layer prevents reaction of the substrate GaAs with the Cl2beam. A quantitative model of the ion‐assisted reaction was proposed and compared with the data.