Free clusters produced by a co-expansion of argon and krypton have been studied by means of core level photoelectron spectroscopy. In this study we have determined: (i) the produced clusters are heterogeneous; (ii) the radial distribution of the two atomic species in the cluster differs. The proposed structure model places the majority of the krypton atoms in the bulk, leaving the argon atoms to be found more frequently on the cluster surface. The suggested structure is qualitatively discussed using the cohesive energies of the two-components. (C) 2004 Elsevier B.V. All rights reserved.