(Ni, Co)3Al4 is a ternary phase in the Al-Ni-Co system, the homogeneous range at room temperature being 55-58.5 A/0 Al, 26-35 A/0 Ni, and 10-15.5 A/0 Co. It belongs to the cubic system with space group Oh10-Ia3d. There are 112 atoms per unit cell, the lattice spacing at room temperature being α = 11.3962?.This structure may be considered as a superlattice built up by stacking together 64 CsCl type fundamental structural units. Among these structural units, there are 16 ‘Centre' positions left vacant in order. They occupy the 16(b) equivalent positions forming 16 octahedral voids. The ‘Corner' positions 16(α) and 48(f) are all occupied by Al atoms, while the ‘Centre' positions 48(g) are occupied at random by Ni and Co. Due to the existence of the voids, the positions originally at cube corners and centres are displaced somewhat, the parameters being xf = 0.010, xg = 0.369.Prom the homogeneous range of this alloy phase and the distribution of the various atoms in the structure, the ideal stoichiometric formula of this alloy phase may be written as (Ni, Co)3Al4. There are 16 formula weights per unit cell.