Measurements have been made, for the first time, of the thermal changes accompanying the step-by-step magnetization of a number of ferrites. The method was that used with metals by Bates and his co-workers, with the added improvement that the thermal deflections were recorded via a split photocell amplifier.Measurements were made on certain mixed ferrites of the nickel-zinc series. The results showed that the major part of the heat changes can be explained satisfactorily in terms of the changes of intrinsic magnetization with field. In some of the specimens a slight irreversible heating was observed, it therefore appears that domain rotational effects make a very small contribution to the thermal changes. An anomalous, low field effect previously found in certain metals occurred in some of the ferrites. This effect is not yet fully understood, although some qualitative suggestions are made to account for it; it is concluded that the observed thermal effects in ferrites can be explained on the same lines as those for metals.