A series of experiments are described on self-reversal of magnetization in a rock containing pyrrhotite. This self-reversal was produced by heat-treatment at about 320°c. Two unusual effects were observed. First, in suitable conditions the direction of magnetization could be made to change sign twice on cooling. Second, after appropriate heat-treatment the reversal mechanism became obliterated; leaving the specimen self-reversed, but containing only a single magnetic material. The second effect is of considerable interest for palaeomagnetism.