Ge surface segregation during Si 1− x Ge x gas-source molecular beam epitaxy on Si(0 1 1) was investigated using in situ D 2 temperature programmed desorption (TPD). The Si 1− x Ge x (0 1 1) layers, x=0–0.20, were grown from Si 2H 6/Ge 2H 6 mixtures at temperatures T s ranging from 475°C to 750°C. Immediately following film growth, the samples were quenched and exposed to atomic D at 250°C until saturation coverage. TPD spectra from Si 1− x Ge x (0 1 1) consist of five second-order peaks due to D 2 desorption from, in order of decreasing temperature, Si rest-atom and adatom monodeuterides, Si dideuteride, and Ge rest-atom and adatom monodeuteride phases. Temperature-dependent Ge surface coverages θ Ge, determined from the TPD results, increase sharply with x ranging, at T s=550°C, from 0.27 ML with x=0.04 to 0.74 ML for layers with x=0.20. For a given film composition, θ Ge decreases with decreasing T s due to the corresponding increase in the fraction f Si,H of Si surface atoms terminated with H. From these data, we find that the Ge segregation enthalpy Δ H s for Si 1− x Ge x (0 1 1) varies from −0.18 eV at T s=750°C ( f Si,H<0.003) to −0.09 eV at T s=475°C ( f Si,H=0.22).