An internally consistent model has been developed to describe the partitioning of Fe2+-Mg among olivine, ilmenite, and spinel in the system Fe-Mg-Ti-Si-O that is also consistent with the Fe-Ti exchange and oxidation of ilmenite-magnetite. Additional Fe-Mg exchange experiments between olivine and ilmenite were conducted at high temperatures to reduce the amount of extrapolation among previous calibrations. The ilmenite model is based on an asymmetric multicomponent Margules solution. The solution models for spinels are based on a Taylor series expansion with (1) a modified Akimoto-type cation distribution and (2) a model that incorporates site mixing of the cations. Both adequately describe the activity-composition relations of the spinels given existing data. In addition to providing thermometric information, the Fe2+-Mg partitioning between olivine and oxide can be used to obtain qualitative T-fO2, paths for one oxide + olivine + quartz assemblages.