Granular Co-Ag alloy films elaborated by molecular-beam epitaxy at three temperatures: 20, 250 and 450 degrees C, have been investigated. The influence of substrate temperatures on the growth and the structural properties is studied, using reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) techniques. For all growth temperatures, the in situ RHEED patterns of the Co-Ag alloys exhibit an in-plane six-fold symmetry, which indicates that the Co and Ag deposits are in epitaxy with the Ru buffer surface. At 20 degrees C, the RHEED patterns show a unique lattice with diffuse lines, which is a feature of a rough surface. However, at 250 degrees C, the RHEED patterns show double lines, which indicate the existence of two lattices, associated with Ag and Co metals. RHEED analysis indicates that the Co lattice constant relaxes to its bulk value after a critical thickness of 20 angstrom, while the Ag has already relaxed at the growth beginning. XRD studies give evidence of a strongly textured alloy films, along the growth direction. It is found that, at 20 degrees C, Co impurities are embedded in the Ag matrix. However, in the range 250-450 degrees C, the system segregates and gives the formation of chemically pure entities of Co and Ag. (c) 2004 Elsevier B.V. All rights reserved.