Production of solar cells has increased rapidly in recent years, and metallurgical production of solar grade silicon (SoG-Si), has increased compared with more expensive processes such as Siemens process and Cz processes. It is important to understand the thermodynamic behavior of phosphorus in slag for optimized refinement of SoG-Si. In this study, the thermodynamic behavior of phosphorus in the CaO-SiO(2)-CaF(2) slag system was investigated at 1773 K for various oxygen potential and slag composition. In addition, evaporation of phosphorus in the reduction process was observed during slag refining by addition of H(2) gas. Experimental results showed that the stability of phosphorus in slag depends on both the O(2-) (basicity) content of the slag and the solubility of Ca in silicon, and the dissolution mechanism of phosphorus into slag was derived. Results confirmed that phosphorus and calcium in molten silicon were also removed by acid leaching. The effective removal of phosphorus for SoG-Si using several refining processes was evaluated quantitatively. (C) 2011 Elsevier B.V. All rights reserved.