The effects of irradiation with Ge⁺ and Ar⁺ ions at elevated temperatures on the relaxation behavior of pseudomorphic Si_0.79Ge_0.21/Si heterostructures have been compared. It was found the strain relaxation in the structures implanted with Ge⁺ at 400°C started already upon implantation, post-implantation thermal annealing of this sample resulted in considerably higher degree of relaxation than that in control (non-implanted) samples as well as in samples implanted with Ar⁺ both at 230 and 4000°C and with Ge⁺ at 230°C. This result points to a dramatic influence of both the implantation temperature and ion species on relaxation behavior of the ion-irradiated heterostructure. Two possible mechanisms for this effect are discussed.