The substitutional fraction,fsof Co implanted into Al single crystals depends on the substrate temperature and on the Co concentration. Implantation at 293 K reveals an anomalous increase offsat low concentrations and reaches maximumfsvalues of 0.40 ± 0.04 for concentrations between 0.12 at.% and 0.4 at.% Co. At 77 Kfsis 0.63 ± 0.04 for concentrations between 0.08 at.% and 1 at.% Co. The non-substitutional Co fraction, 1-fsis governed by Co-vacancy complexes which form within the collision cascade and by additional trapping of mobile vacancies at 293 K. The decrease offsat high Co concentrations, which was observed for both the 77 K and the 293 K implants, is due to a transition into a disordered, probably amorphous phase. Implanting similar Co-concentrations, the disordered phase fraction is larger for 77 K implants.