Submarine basalts have been heated in water in the laboratory to study the changes of magnetic properties with oxidation of the titanomagnetite. Before heating, the titanomagnetites were cation deficient with z ~ 0.1. With prolonged heating the degree of cation deficiency, as determined from the Curie temperature, increased Further heating in an attempt to produce titanomagnetites with z > 0.6 caused phase-splitting. This phase-splitting was sometimes accompanied by a self-reversal of magnetization. From the results of the experimental alterations we are able to predict the decrease of magnetization with distance from a spreading ridge.