Due to the use of phosphorus-containing nickel substrates in microelectronics, understanding of their reaction with Sn-based solders and knowledge of the corresponding reaction products is highly important. Therefore the ternary Ni-P-Sn system was investigated experimentally using x-ray diffraction (XRD), scanning electron microscopy (SEM) + energy dispersive x-ray spectroscopy (EDX)/wavelength-dispersive x-ray spectroscopy (WDS), and differential thermal analysis (DTA). Sample preparation was done by alloying powders of the starting components contained in alumina crucibles in evacuated quartz tubes. In this study the phase equilibria in the Ni-rich part of the ternary Ni-P-Sn system are described in the form of three partial isothermal sections at 550°C, 700°C, and 850°C. A total of five ternary compounds exist in the Ni-rich part, and the binary Ni3Sn2 HT phase was found to dissolve 17.6 at.% P at 850°C. This well agrees with literature reports, while a different extent and shape of the large homogeneity range of this phase were found. Between 850°C and 700°C phase equilibria change significantly due to the formation of the ternary phases Ni10P3Sn5 (T3), Ni13P3Sn8 (T4), and Ni2PSn (T5). © 2009 TMS.