Diffusion of antimony in relaxed Si1−xGex alloy layers grown by molecular beam epitaxy has been studied as a function of the composition for 0≤x≤0.5. The diffusivity of antimony was found to increase with the Ge alloy content, and the extracted activation energies for diffusion were found to decrease with increasing germanium content in the investigated composition range. The activation energies, however, are significantly higher than the corresponding activation energies obtained by an extrapolation between the activation energy in pure silicon and germanium.