Phase relations in the system NiO–TiO2have been determined by heating oxide mixtures in air at selected temperatures in the range 1300° to 1750°C for sufficient periods of time to attain equilibrium, followed by rapid quenching to room temperature. The phases have been characterized by optical microscopy, X‐ray diffraction, and electron microprobe analysis. The most striking feature is the presence, above 1430°C, of a spinel‐type phase that decomposes below this temperature to a mixture of remnant spinel, NiO of periclase‐type structure, and NiTiO3of ilmenite‐type structure. There are two peritectic points in the system, one at 1730°C where spinel, NiO, and liquid coexist in equilibrium, and one at 1610°C where spinel, NiTiO3, and liquid are the coexisting phases. A eutectic is present at 1570°C, with NiTiO3, rutile, and liquid coexisting in equilibrium. Rapid transformation of the spinel phase, even during rapid quenching, imposes uncertainties on the interpretation of the experimental data obtained, but the equilibrium‐phase relations are deduced essentially as shown in the phase diagram presented. Results of a small number of calculated activities of NiO in oxide‐phase assemblages involving the spinel phase at high temperatures (∼1500°C) lend support to the interpretation of the phase relations as p