Summary: A hypothesis for the mechanism of oxidation in titanomagnetites is first put forward on the basis of new oxidation data for pure magnetite and the new model for cation distribution in titanomagnetites. Contrary to previous claims by other workers, it is shown that single-phase spinel oxidation products of titanomagnetites can only be produced under very exacting circumstances. Lowtemperature magnetic measurements on such synthetic, oxidized materials were made to show the formation of magnetite by the decomposition of the metastable oxidation products of titanomagnetites on prolonged heating at moderate (∼ 390° C) temperatures. This magnetic method of detection is more sensitive than that of normal X-ray diffraction. It is concluded that previous claims of an increase in magnetization due to the formation of synthetic single-phase, highly oxidized titanomagnetites must be due to the formation of undetected magnetite.