A theoretical model of equilibrium phase relations in the system Fe2O3–FeTiO3 is presented. This model is based on the single prism approximation in the cluster variation method and includes both chemical and magnetic contributions to the free energy of mixing. The inclusion of a magnetic degree of freedom and magnetic interaction parameters makes it possible to assess the effect of magnetic ordering on the compositions of coexisting titanhematite and hemoilmenite phases, and this effect is shown to be considerable. Two tricritical points are predicted to occur in the Fe2O3–FeTiO3 phase diagram: one at which an Fe–Ti order–disorder transition pierces the peak of a two-phase field; and a second at which the two phase field is intersected by an essentially antiferromagnetic transition. Below this latter point, the two-phase field is predicted to bulge out, towards Fe2O3, and it is argued that this feature should be useful in geothermometry.