The diffusion of ion‐implanted phosphorus in silicon during rapid thermal annealing has been studied. Depending on the implant dose, we find two distinct kinds of diffusion behavior. For low dose (1×1014cm−2) P+‐implanted Si, a profile redistribution is observed which becomes observable at 900 °C for annealing times of 10 s, but which is temperature independent in the range 800–1150 °C. This initial redistribution is much more rapid than conventional diffusion coefficient data would predict. For high dose (2×1015cm−2) P+implanted and short time (10 s) annealed Si, the dopant profile broadening is strongly temperature dependent. The experimental profiles are in this case in agreement with concentration enhanced diffusion profiles.