Phase relations in the system FeO-Fe2O3-TiO2 at 1300° C. have been deduced from the relation between oxygen pressure and composition. This relation was determined by heating samples, made up from Fe2O3 and TiO2, in flowing mixtures of CO2 and H2, and measuring the weight loss of each sample for each gas composition with a thermobalance. This relation is shown by lines of constant oxygen pressure (gas composition) plotted on the composition triangle FeO-Fe2O3-TiO2. Phase relations were deduced from the nature of these lines. Essential features of the system at 1300° C. are complete solid solutions between magnetite and ulvöspinel (2FeO·TiO2), between hematite and ilmenite (FeO·2TiO2), as well as between the compositions Fe2O3· TiO2 and FeO·2TiO2. Precise compositions of coexisting solid solutions are given by end points of lines of constant oxygen pressure (tie lines) which connect solid solutions. The maximum amount of vacancies in the place of cations in the spinel solid solution at 1300° C. is about 2 per cent, regardless of the titanium content.