As-rich GaAs (110) is prepared by ion bombardment and annealing, followed by chlorination and reannealing. The surface is then reacted at room temperature with Cl-2 gas and examined with soft x-ray photoelectron spectroscopy of the Ga and As 3d core levels. After low exposures (<5x10(4) L), the surface appears to passivate with half a monolayer of C1 adsorbed, primarily as AsCl. Following sufficiently large (>5x10(4) L) exposures, however, the surface begins to etch, as indicated by the continuous uptake of chlorine and the formation of As and Ga chlorides. After the largest exposures, the distribution of As chlorides still favors the monochloride, whereas the Ga chlorides favor GaCl2. It is proposed that the heavily reacted surface is covered with -AsCl-GaCl2 treelike structures. The addition of C1 to form GaCl3 from GaCl2 is identified as the rate-limiting step in the overall etching reaction.