The natural occurrence of non-stoichiometric (cation deficient) titanomagnetites in continental and submarine igneous rocks is now becoming well established. The existence of such materials must indicate that, given geological time, coarse grained titanomagnetites may oxidize to cation deficient spinels at low temperature, such changes being impossible in laboratory times and temperatures. In the present paper the production of non-stoichiometric titanomagnetites is described using the technique of pregrinding titanomagnetites in a water slurry followed by oxidation in air at temperatures <300 °C. The influence of grinding on the oxidation product is shown directly by differential thermogravimetric (DTG) analysis. The reaction kinetics are also studied by gravimetric and magnetic methods. It is found that, below about 300 °C, all compositions represented by points in the Fe 3O 4Fe 2TiO 4Fe 2TiO 5Fe 2O 3 quadrilateral of the FeOTiO 2Fe 2O 3 ternary diagram may exist as spinels. The stability of these materials on heating above 300 °C in evacuated capsules has also been studied: γ-Fe 2O 3 produced by oxidation of pre-ground Fe 3O 4 inverts to α-Fe 2O 3, but the presence of Ti 4+ ions in all other cation deficient spinels results in a variety of inversion products depending on the composition of the spinel phase before inversion. Titanium appears to be ineffective in stabilising the cation deficient structure.