The application of ion-beam mixing to the doping of silica films with metals either in solution or in the form of nanoclusters requires a preliminary investigation of the mixing mechanisms. This was undertaken by means of RBS and XRD for Au, Pt, W, Ag and Cu films mixed with silica by 4.5 MeV Au or 200 keV Ar ions, at 100 or 300 K. In all cases metal atoms migrated preferentially inwards. At room temperature, the radiation enhanced diffusion of Au, Pt, Ag and Cu in the silica matrix competed with their coalescing into coarser particles, under the effect of interfacial stresses. The mixing rate was proportional to the ion dose or to its square, depending on the system and temperature. This unusual quadratic law is attributed to the formation of solute metal-vacancy complexes.