Nanostructured thin films were prepared from mixed Co-Ag clusters preformed in the gas phase, in order to improve the magnetic properties previously obtained for pure superparamagnetic Co clusters embedded in miscible matrices. In this paper, we focus on the morphology of such mixed nanoparticles to confirm a core-shell model. Electron diffraction has shown that a Co-Ag segregation occurs but preliminary local structure and magnetic results indicate that the Ag-shell around the Co-core is not perfect, some Co atoms being in direct contact with matrix atoms. (C) 2003 Elsevier B.V. All rights reserved.