A pseudomorphic Ge0.12Si0.88 film 265 nm thick grown on a Si(100) substrate by molecular beam epitaxy was implanted at room temperature with a dose of 1.5 × 1015 cm2 of 100 keV P ions. The projected range of the ions is about 125 nm, which is well within the film thickness. Only the top portion of the Ge0.12Si0.88 layer was amorphized by the implantation. Both implanted and non-implanted samples were subsequently annealed in vacuum for 30 minutes from 400 °C to 800 °C. Values of electron Hall sheet mobility and concentration in the implanted Ge0.12Si0.88 epilayer were measured after annealing. The solid phase epitaxial regrowth is complete at 550 °C, where the implanted phosphorus reaches approx. 100% activation. The regrown Ge0.12Si0.88 layer exhibits inferior crystalline quality to that of the virgin sample and is relaxed, but the non-implanted portion of the film remains pseudomorphic at 550 °C. When annealed at 800 °C, the strain in the whole epilayer relaxes. The sheet electron mobility values measured at room temperature in the regrown samples (Tann ≥ 550 °C) are about 20% less than those of pure Si.