A theory of magnetic instabilities which qualitatively explains the anomalous thermal properties of invar is presented. Fixed-spin-moment band calculations are used to show the existence of two-spin-states similar to Weiss's 2γ-state model. The two spin-states are connected by magnetic instabilities which are shown to occur near the equilibrium volume in the Fe-Ni invar alloy with approximately 8.6 valence electrons/atom. The consequences of lattice vibrations on the thermal occupation of the two states are examined and it is shown that the low temperature expansion is dominated by a high-spin state, while the high temperature expansion results from a composite occupation of a high-spin and a low-spin state. The initial negative thermal expansion is shown to result from an antiferromagnetic state. © 1989.