The formation of zinc ferrite, from a stoichiometric mixture of zinc oxide and ferric oxide, has been investigated from 600 C to 1150 C, with particular reference to the temperature of preparation of the ferric oxide. Under conditions which parallel those of roasting after removal of the bulk of the sulfur, there is a sharp decrease in the reactivity of the ferric oxide if the temperature of preparation exceeds about 675 C. There is no change in ferric oxide at this temperature, except for the Curie point at 678 C. The reaction is controlled by the diffusion of ferric oxide through the ferrite layer. The activation energies for the low and high temperature ferric oxides are 110,000 and 57,000 calories per mole respectively.