Lime particles (0.5 to 1 mm) were added by dropping to CaO–Fe_tO–P₂O₅ slag at 1 400°C, followed by stirring and the reaction products near a CaO particle/slag interface were determined by a microprobe analysis (EPMA-EDX). The 3CaO·P₂O₅ and 4CaO·P₂O₅ phases were crystallized depending on the amount of added CaO particles which are rapidly dissolved after the contact with a molten slag. The phosphorous distribution ratios between calcium phosphates and CaO–Fe_tO slag obtained at temperatures between 1 400 and 1 570°C are higher than those between 2CaO·SiO₂–3CaO·P₂O₅ solid solution and CaO–SiO₂–Fe_tO slag. Lime particles (0.3 to 0.5 mm) are added to CaO–SiO₂–Fe_tO–P₂O₅ slag at 1 400°C and the reaction products near a CaO particle/slag interface are determined by EPMA-EDX. The 2CaO·SiO₂–3CaO·P₂O₅ solid solution layer is formed on the surface of a CaO particle and thereafter the CaO–Fe_tO slag layer is formed between a CaO particle and 2CaO·SiO₂–3CaO·P₂O₅ solid solution layer which is surrounded by CaO–SiO₂–Fe_tO–P₂O₅ slag. It is also experimentally confirmed that the meso-phase slag refining is superior to conventional slag refining and the discrepancy between the theoretical and experimental results obtained by the meso-phase slag refining is discussed.