High-dose arsenic and phosphorus ion implanted silicon was annealed either by a CW CO2 or a pulsed Nd:YAG laser creating supersaturated dopant concentrations up to 3·1021 cm−3. The relaxation of these metastable electrically active atoms was investigated during thermal post-annealing at temperatures between 600 and 1000°C for times between 3 and 106 s. In heavily doped samples which contain residual damage after laser annealing, a very fast first relaxation phase is observed followed by a much slower second phase. In samples without residual damage only this second slower phase is seen. Carrier concentration profile measurements show that the saturation concentration after the relaxation depends only on temperature and corresponds to the concentration in thermal equilibrium. Using reaction kinetics a cluster model is proposed which demonstrates that in As doped layers the most probable number of As atoms in one cluster depends on temperature (4 As atoms at 700°C, 3 As atoms at 800 - 1000°C). In P doped layers the most probable clusters contain 3 P atoms at temperatures between 700 and 900°C.