A model is proposed to account for the metallurgical and physical properties of f.c.c. Fe−Ni alloys, including the Invar anomalies around 34 at.% Ni. Microscopic inhomogeneity due to short range atomic ordering on the basis of the FeNi 3 compound is assumed to be a dominant feature of the alloy system. Regions of both f.c.c. Fe and FeNi 3 are present in alloys in the invar region. The electronic state of only the Fe atoms, and hence their volume, magnetic moment, and sensitivity to external variables, is further assumed to be a function of the number of Ni neareast neighbours, which in turn depends on such variables as cold work. With these assumptions, the properties of the annealed alloys above the Curie point are developed, the phase diagram and metastability of the alloys is explained and the Invar anomalies below the Curie point are accounted for. Furthermore, the model explains the unusual low temperature properties of these alloys as due to exchange coupling between ferromagnetic FeNi 3 regions and antiferromagnetic Fe regions. An extension of the model is made to other binary alloy systems that exhibit Invar-type anomalies.