Magnesium ferrite varies in cell size, density, and refractive index, according to the temperature from which it was quenched, over the range 350° to 950°C. There is no evidence for long-range ordering, but the change toward a more inverse structure with decreasing temperature is toward a more ordered state. The DTA tracing shows a strong exothermic peak at 450°C. The effect of the cooling rate on the thermal contraction of magnesium ferrite is discussed.