Experiments are described which demonstrate that during the isothermal dissolution of GaAs wafers into undersaturated Ga-Al-As solutions a graded layer of the alloy (GaAl)As is present on the surface. Such a layer can only be formed by solid diffusion. A kinetic analysis of the situation using a value of 5 x 10-12 cm2 sec-1 for the diffusion coefficient of Al in the solid gives a good agreement with the experimental results. The fact that a layer is formed during dissolution is counter to a number of recent publications on the subject, while the measured diffusion coefficient is very different from that measured in bulk MBE material. The significance of obtaining an understanding of this situation is relevant to the formation of all heterojunctions by near-equilibrium processes. © 1980, The Electrochemical Society, Inc. All rights reserved.