Abstract Experimental studies of the standard molar Gibbs free energy of formation of NiWO 4 have been carried out using a solid-state galvanic cell involving (zirconium oxide + calcium oxide) electrolyte in the temperature range T = 1034 K to 1317 K. The galvanic cell used can be represented as -Pt|{(1 - δ)Ni + δW}(f.c.c.), NiWO 4 (s), WO 2.72 (s)|(1 - x )ZrO 2 + x CaO|NiO(s), Ni(s)|Pt + , where δ « 1. The e.m.f. against temperature plot shows a break at T ≈ 1167 K, indicating a possible phase transformation occurring in the ternary mixture. These studies were complemented by X-ray-diffraction studies of some ternary mixtures equilibrated at T = 1073 K and 1273 K. A thermodynamic assessment of (nickel + tungsten + oxygen) was carried out as part of the present work, incorporating the present results along with those available in the literature for the ternary as well as the three binaries involved. The isothermal sections at T = 1073 K and 1273 K are presented.