The redistribution of Be and Mg implants upon post‐implant annealing is studied in order to evaluate the influence of implant damage on the diffusion process. Rapid uphill diffusion is observed in the peak of Mg implants in GaAs, whereas Be implants show only uniform, concentration‐dependent diffusion. This behavior is explained by the substitutional‐interstitial‐diffusion mechanism and computer simulations of damage‐produced point defects. In the region of uphill diffusion, the dopants diffuse from areas of excess interstitials toward areas of excess vacancies. A critical concentration of point defects is necessary to initiate uphill diffusion. Uphill diffusion can be induced in Be implants by co‐implanting with a heavier element such as Ar.