Alloys of nickel and arsenic containing up to 60% As by weight have been studied by means of room temperature and high temperature Debye-Scherrer diagrams. Three compounds have been identified: Ni5As2, Ni12−xAs8 (maucherite), and NiAs (niccolite). The first of these is homogeneous from Ni5As2 to Ni4.8A2 at room temperature, and to Ni4.6As2 above 250 °C., while the latter is homogeneous from NiAs to Ni0.95As. Contrary to expectations the stability region of the compound Ni12−xAs8 is very narrow, and occurs at Ni11As8 rather than at Ni3As2. Evidence is presented in support of Hansen's contention that this compound has an incongruent melting point. Alloys in the region corresponding to Ni4.6As2 undergo two transitions below 200 °C, one of which is martensitic and produces a metastable phase, while the other is believed to result in the formation of a new compound, as yet unidentified. The diffraction patterns are discussed in some detail.