Japan relies on imports for almost the entire amount of phosphorus, which is indispensable for human life and industrial materials. Therefore it is required to recover phosphorus from dephosphorization slag and sewage sludge which are promising unutilized phosphorus resources. Aiming wholesale recovery of phosphorus from the slag and sludge, a fundamental study on condensation of phosphorus in solid phase through high temperature phase separation was carried out. The experiments at 1573 K on principal [CaO–SiO₂–P₂O₅] ternary model sample and Al₂O₃ and Fe₂O₃ added therein showed unsaturation with CaO and precipitation of solid 3CaO·P₂O₅ as the phosphorus-concentrated phase in all the sample. The contamination of the other components in 3CaO·P₂O₅ was low, especially less than 1 mass% for Al₂O₃ added samples. The liquid phase was also formed with Al₂O₃ or Fe₂O₃ addition, and the liquidus composition in quaternary samples were consistent with those appeared in [CaO–SiO₂–Fe₂O₃] and [CaO–SiO₂–Al₂O₃] ternary diagram. P₂O₅ content in the quaternary liquid phases were lower in Al₂O₃ added samples than those in Fe₂O₃ added samples. Such difference was discussed in terms of optical basicity and CaO activity for liquid phase. Due to the coexistence of solid SiO₂ phase at Fe₂O₃ addition, the SiO₂ content in liquid phase became lower compared to Al₂O₃ addition, and led to higher optical basicity, which would allow higher P₂O₅ capacity in liquid phase. These considerations strongly suggest that an effective control of composition would be key technology for the phosphorus recovery through condensation and separation of phosphorus concentrated phase.