Phosphorus segregation at Si/SiO2 interfaces was examined experimentally and by simulation, for phosphorus implanted at doses of 2×1013 and 7×1013 cm-2. Reversible loss of implanted phosphorus was observed during annealing due to interface segregation. Two stages of dose recovery were observed in samples that were implanted at 7×1013 cm-2. The surface concentration remains almost constant with rapid dose recovery during the early stage of annealing. The rate of dose recovery declines with the decay of the surface concentration after long-term annealing. Samples with implantation at 2×1013 cm-2 exhibit only slow dose recovery. Slow dose recovery at low surface concentration was captured by simulation based on a conventional interface trap model. An interface clustering model was developed to simulate rapid dose recovery with an almost constant surface concentration. The interface clustering model assumes clustering reactions between phosphorus atoms at the interface while the interface trap model considers phosphorus trapping by individual interface defects. The difference between the segregation mechanisms is evidenced by the fact that the extracted segregation energy of the interface trap model greatly exceeds that of the interface clustering model.