We have investigated how secondary phases appear and disappear upon increasing europium concentration from 1.5 to 16.2 atom % in standard strontium borophosphate phosphate phosphor, Sr6BP5O20, using X-ray diffraction and photoluminescence and photoluminescence excitation measurements both at room temperature and at 80 K. In standard phosphor, a minor phase of strontium phosphate, Sr-3(PO4)(2), was detected besides the main component Sr6BP5O20. With increasing Eu concentration, the phase Sr-3(PO4)(2), disappears and new phases of Sr3Eu(PO4)(3), SrBPO5, and Sr2P2O7 are formed. Sr3Eu(PO4)(3) is observed to be the major phase in 16.2 atom % samples. It contains only trivalent europium. An attempt has also been made to understand the origin of these secondary phases and their significance for the performance of this luminescent material. This study shows how X-ray diffraction technique and photoluminescence spectroscopy can be used in a complementary manner to explore the phase composition of the materials. (C) 2008 The Electrochemical Society.