Si1-xGex thin films were grown on silicon substrates by ion beam sputter deposition (IBSD) in an ultrahigh vacuum system. The film stress was determined from the change in deflection curvature of the substrate after deposition and was studied as a function of different parameters: growth temperature, composition (0% ≤ × ≤ 60%) and annealing conditions. The stress is mostly compressive and greater than that observed in films prepared under similar deposition conditions by molecular beam epitaxy (MBE). We observed an additional contribution of about 1 GPa independent of the film composition and thickness. The evolution of stress upon post-growth annealing conditions was followed. A plot of ln(σ) as a function of 1/kTa reveals a thermally activated process. All samples (0% ≤ × ≤ 8%) yield a similar value for the activation energy (QSR=0.1-0.3 eV). This value is considerably lower than that obtained for epitaxial films deposited by MBE or LRP-CVD. We speculate that the stress observed in IBSD layers is due to the effect of bombardment of the growing film by energetic particles. Measurements by Rutherford backscattering spectrometry show that the concentration of rare gas (xenon) incorporated in the films is too low to account for the stress. However, it is found that backscattered ions and (or) sputtered atoms that strike the growing film may have energies large enough to displace atoms and lead to compressive strain. To support this model quantitatively, computer calculations were carried out using an advanced version of the Monte Carlo transport of ions in matter, TRIM.