Solid state phase equilibria in the ternary Ni---Al---As diagram were established at 800 °C. The experimental techniques used to elaborate the phase equilibria were X-ray diffraction (XRD), electron probe microanalysis (EPMA) and scanning electron microscopy (SEM). Three ternary phases, which crystallize in hexagonal symmetry and are all structurally derived from the NiAs type, were evidenced in the Ni-rich part of the diagram. Among them, two ternaries labelled as A and D phases, by comparison with the isostructural ternary phases in the Ni---Ga---As diagram, reveal fully disordered structures; in contrast, the ternary B phase shows a hexagonal superlattice (a√3, 3c), which denotes an ordered structure. Very limited solid solubilities were measured in the binary constituent Ni---Al and Ni---As compounds, with the exception of NiAs which showed a homogeneity range with an Al-rich limit corresponding to the formula NiAs_0.7Al_0.3. Neither nickel nor ternary phases are in thermodynamic equilibrium with AlAs, in contrast with the binaries NiAl, Ni₂Al₃ and NiAs which are. The bottom part of the experimental diagram differs from the theoretical one deduced from estimated thermodynamic data on Ni---Al and Ni---As binaries using Miedema's model, but is in agreement with interfacial reaction studies which concluded that NiAl and substituted NiAs are the stable phases when Ni thin films are reacted to completion on AlAs. Finally, the influence of the IIIa element (Ga or Al) in the Ni---IIIa---As system was considered through a comparative study between Ni---Ga---As and Ni---Al---As diagrams. The pseudoternary Ni---(Ga,Al)---As system was experimentally estimated for the atomic composition (Ga_0.7Al_0.3), which is the value usually chosen for the elaboration of GaAs/(Ga,Al)As heterostructures with Ga_1−xAl_xAs exhibiting a direct gap of highest energy.