The effects of surface temperature (T-s) and surface structure upon the passivation and etching of GaAs(100) and GaAs(110) surfaces by C1(2) have been studied. The Ga-rich GaAs(100) Ga-c(8X2) and the stoichiometric GaAs(110) (1X1) surfaces form stable ordered monochloride overlayers when exposed to Cl-2 at 300 K. The ordered overlayers formed are AsCl on the GaAs(110) (1X1) surface and GaCl on the GaAs(100) Ga-c(8X2) surface. In contrast, the As-rich GaAs(100) As-c(2X8) surface undergoes continuous uptake of Cl-2 under equivalent conditions. All three surfaces exhibit continuous uptake of Cl-2 at surface temperatures above 600 K, where continuous etching is known to occur. In this paper, the continuous uptake of Cl-2 is shown to result from etching of the surface. In addition it is shown that the Ga-rich GaAs(100) Ga-c(8X2) and the stoichiometric GaAs(110) (1X1) surfaces can be induced to undergo continuous etching at surface temperatures of 300 K, if they are first exposed to Cl-2 with surface temperatures above 600 K. This slight pre-etch disrupts the surface order and exposes multiple dangling bonds. The disrupted surface does not allow for a monochloride passivation layer to form; instead, volatile trichloride products form, leading to continuous etching. Therefore, the dissociative adsorption of molecular chlorine on GaAs(100) and GaAs(llO) surfaces involves a competition between etching and passivation in which monochloride passivation is favored only on the well-ordered surfaces.