Sodium borosilicate glasses containing phosphorus were prepared, and the effect of Al2O3 and TiO2 addition on the distribution behavior of phosphorus atoms associated with the phase separation was investigated. After heat treatment, a phase separation by the spinodal decomposition was commonly confirmed by SEM observation. In the additive-free sample, phosphorus was preferentially distributed into B2O3-rich phase after the phase separation, and after adding Al2O3 or TiO2, the amount of phosphorus distributed into SiO2-rich phase increased. It was also suggested that TiO2 had a stronger influence on the phosphorus distribution into SiO2-rich phase than Al2O3. P-31 MAS-NMR revealed that most of phosphorus atoms in the additive-free glass were present in PO4 units consisting of two terminal oxide ions, and the terminal oxide ions decreased and increased after the additions of Al2O3 and TiO2, respectively. It was suggested that this structural change brought about a change in the distribution of phosphorus. (C) 2012 The Ceramic Society of Japan. All rights reserved.