Phase relations in the system Ni-As were determined in rigid silica-glass tubes, in collapsible Au tubes, and by differential thermal analyses. The system includes the well established minerals maucherite (Ni 11 As 8 ), niccolite (Ni 1 + or -xAs), and NiAs 2 polymorphs rammelsbergite and pararammelsbergite. A phase with the composition of Ni 3 As (dienerite) could not be synthesized, and if this phase exists it must be stable only below 200 degrees C. Ni 5 -xAs 2 is stable to approximately 993 degrees C. and has a large variation in its Ni/As ratio. Maucherite, which is essentially restricted to Ni 11 As 8 composition, melts incongruently at 830 degrees + or -5 degrees C. to niccolite plus a liquid. The existence of a metastable form of Ni 11 As 8 appears to be likely. Niccolite, which is stable to 962 degrees + or -3 degrees C., also has a large variation in its Ni/As ratio. The niccolite solvus between NiAs and NiAs 2 is not useful as a geothermometer, however, since it is nearly vertical in the temperature range of geologic interest. The pararammelsbergite-rammelsbergite inversion was found to occur at 590 degrees C. under the vapor pressure of the assemblage when pure NiAs 2 is in equilibrium with niccolite. The inversion temperature is raised 22 degrees C./1000 bars, giving a H of 0.57 Kcal/mole at 590 degrees C. When pure NiAs 2 is in equilibrium with metallic As instead of niccolite, the inversion temperature is approximately 8 degrees C. higher. Investigation of the inversion temperatures of natural specimens of rammelsbergite and pararammelsbergite shows that solid solution of elements such as Fe, Co, and S may lower the inversion by more than 100 degrees C.