In some cases, XPS analysis can be used to determine the reactions that take place at a solid/solution interface. In this study, the behaviour of GaAs in contact with an acidic aqueous solution containing a Keggin-type heteropolyanion—SiMo12O404−—was investigated on the basis of the XPS data. During immersion in the solution, a film composed of elemental As and partially reduced heteropolyanion develops on the GaAs surface. The chemical composition of the surface film indicates that at the GaAs/solution interface the heteropolyanion is reduced whereas GaAs undergoes an anodic dissolution process. The GaAs dissolution rate is proportional to the heteropolyanion concentration. At a given immersion time, the nature of the GaAs surface film is independent of the heteropolyanion concentration, whereas the As0/heteropolyanion ratio and the film thickness depend on this concentration. Moreover, the chemical composition of the film changes by exposure to air, as some elemental As atoms volatilize. Copyright © 2002 John Wiley & Sons, Ltd.