The composition profile of self-assembled InAs/GaAs quantum rings is investigated both experimentally and theoretically. Our study is aimed at obtaining information on unburied rings, which cannot be directly accessed in cross-sectional analysis. Two-dimensional surface chemical maps obtained by X-ray photoemission electron microscopy reveal a non-uniform composition profile with a double structure composed of an In-rich core, corresponding to the central hole of the ring, surrounded by a rim with a stronger In-Ga intermixing. These results are substantiated by an atomistic model which, for a given shape, identifies the composition distribution that minimizes the elastic energy of the system, The good agreement between experiment and theory allows us to identify the minimization of strain energy as the main driving force for the formation of quantum rings. (C) 2008 Elsevier B.V. All rights reserved.