When solid GaAs is placed in an undersaturated solution of Ga, Al, and As, it has been observed that a layer of the solid Ga1−xAlxAs forms on the surface. In the past the presence of this layer has been attributed to a process of regrowth following sufficient dissolution to saturate the solution. On the other hand, an analysis of the kinetics of the situation has suggested that dissolution should be continuous and that the surface layer is formed by solid diffusion. An experiment is reported here in which the solid is forced to dissolve continuously, and a layer of Ga1−xAlxAs of similar thickness to those reported by others has been found to be produced on the surface. Such a layer could not have been produced by regrowth. In order to be produced by solid diffusion, the diffusion coefficient of Al in GaAs must be anomalously high.