In-situ processing requires totally dry approaches to pattern generation to be developed. Of particular interest are beam driven reactions that can be localised without prior masking. This paper considers the adsorption - desorption characteristics of chlorine, dichloroethane and sulphuryl chloride on GaAs(lOO). The surface reactivity in these systems is seen to vary considerably. The implications of this are considered with regard to their use as chemically assisted ion beam and laser photochemical etching gases.