The Curie and Néel points, and other critical points, of alloys of heavy rare earth elements with each other have been determined for 14 binary alloy systems. To a good approximation the Néel points of all of the alloys vary linearly with the power of the average de Gennes factor, J(J+1)(g-1)2, as previously known. The Curie points, however, are definitely characteristic of the alloy system, each system showing a different dependence on concentration. The properties of each system are discussed, and related when possible to the transformations found in the elements and to atomic processes. Special effects are noted when the average 4f electron concentration lies between 10 (Ho) and 11 (Er), where the Stevens factor changes sign. Magnetization processes in several alloys are interpreted according to the model of Kitano and Nagamiya. © 1966 The American Physical Society.