This paper describes the magnetic properties of a group of iron-nickel-cobalt alloys, named “perminvar.” With certain heat treatments these alloys have unusual constancy of permeability and extremely small hysteresis losses at low flux densities, and peculiarly shaped hysteresis loops constricted in the middle as the maximum flux densities of the loops are increased. Methods of preparing and heat treating the alloys are described, limits of composition, and changes in the magnetic properties with composition and with different heat treatments are illustrated. A theory of constitutional changes effected by heat treatment and responsible for the unusual magnetic properties is suggested.