[en] Many studies have suggested that Fe-Ni and Fe-Ni-Cr alloys of ca. 35 at. % Ni are susceptible to decomposition into two FCC phases. We herein model the kinetics and thermodynamics of spinodal decomposition, nucleation, growth, and coarsening of these alloys. Prediction of the scale of a microstructure formed by growth or coarsening may be done with some confidence. The absence of a model for surface energy and gradient energy in systems with negative departure from ideality reduce us to little more than guesses for the kinetics of nucleation and of spinodal decomposition. We combine our calculations with those of others and studies of terrestrial and celestial decomposition to suggest a miscibility gap for Fe-Ni alloys. The gap is very narrow at high temperatures but is broadened at low temperatures by magnetic effects. 29 refs., 15 figs