The influence of matrix and substrate on the morphology of three Pt nanocermet thin films has been studied by transmission electron microscopy (TEM), X-ray specular reflectivity and secondary ion mass spectrometry (SIMS). The TEM measurements clearly evidence the presence of metallic nanoparticles inserted in an amorphous insulator matrix. The structure of the thin films in the z-direction, normal to the surface of the films, is then analysed by specular X-ray reflectivity. Using two different models to analyse the data, it is shown that although the size and the mean separation of the particles does not depend much on the nature of the insulator matrix, the substrate and the insulator matrix play a key role at the film-substrate interface. These conclusions are reinforced by the SIMS analysis which shows that the diffusion of metallic particles in the substrate is important in presence of alumina matrix.