Metastable Cu-Mo thin films have been deposited on quartz substrates by ion beam Sputtering. The structural and mechanical state characterisation in these as-deposited films have been performed using X-ray diffraction. The size of the coherently diffracting domains is small (1.5-2.0 nm) and the micro-strains are very large (0.3-1.9%). The value of these parameters depends on the Mo concentration. The sin(2) psi method was applied to extract stresses and the stress-free lattice parameter a(0) in the Cu-30 Mo-70 films. In-plane stress is compressive and its amplitude is very high (4.5 GPa). The presence of such high intrinsic compressive stresses is related to lattice distortions induced by an ’atomic peening’ effect and Ar interstitials during the deposition process. After thermal annealing, one observes the demixing of the solid solution (a(0) evolution) and an inversion of stress sign (compressive to tensile).