The etching of the (100) face of gallium arsenide with molecular iodine has been studied at I2 pressures between 0.10 and 1.25 Torr and in the temperature range from 270 to 330 °C. GaAs was found to be etched continuously at rates between 0.05 and 1.70 μm min-1 under these conditions. Although the etch rate appears to be first order with respect to I2 at pressures below 0.3 Torr, the order decreased at higher pressures. The data was analyzed in terms of two mechanisms, which have been proposed for the etching of semiconductors by halogen molecules. These are the ‘‘reversible dissociative adsorption’’ (RDA) mechanism, and the ‘‘surface site saturation’’ (SSS) mechanism. In the RDA mechanism the dissociative adsorption, which explains the nonlinear pressure dependence, occurs irreversibly at low pressure giving rise to first order kinetics with a first order rate constant (k1) which can be expressed in the following Arrhenius form: k1=104.7±0.2 μm min-1 Torr-1 exp[-(55±2) kJ mol-1/RT]. At higher pressures, the dependence on I2 changes to half order and the composite half order rate constant (k1/2) can be represented by the equation: k1/2=106.5±0.9 μm min-1 Torr-1/2 exp[-(69±10) kJ mol-1/RT]. On the other hand, the SSS mechanism involves the reversible physisorption of I2 on the GaAs surface (governed by an equilibrium constant K) followed by a rate controlling reaction of this physisorbed species leads to products (governed by the rate constant k4). The values obtained for these two constants are: K=10-4.8±0.7 Torr-1 exp[+(49±7) kJ mol-1/RT], and- - k4=109.2±0.4 μm min-1 exp[-(96±5) kJ mol-1/RT]. The etching is isotropic, and the major products are GaI3 and AsI3.