The magnetization of two alloys, as affected by heat treatment in a magnetic field at various temperatures, is examined in some detail in order to elucidate the nature of the accompanying changes which result in some cases in a 30‐fold increase in maximum permeability. Theexperimentsshow that these alloys (one containing approximately 35 percent iron and 65 percent nickel, the other 20 percent iron, 60 percent cobalt and 20 percent nickel) can be effectively heat treated in a magnetic field of 10 oersteds if the temperature is above 400°C and below the Curie point of the alloy. The time during which the magnetic properties change has been measured at different temperatures and is found to vary according to the equation &tgr;=AeW/kT. The experiments areinterpretedin terms of the domain theory of ferromagnetism. The changes which occur are due to the relief of magnetostrictive stresses which arise when the material becomes ferromagnetic upon cooling through the Curie point or when an external magnetic field is applied, and the relief comes about by plastic flow or diffusion in the separate domains. The values ofA(about 10−12second) andW(2.1 electron volts) are the same as those determined by Bragg and Williams for the above equation which also gives the time necessary for the establishment of a superstructure in alloys. The relation between the two processes, establishment of superstructure and the relief of magnetostrictive strains, is pointed out.