In oxidic lattices the Co3+(3d6) ion has been found both in the diamagnetic low‐spin state and in the paramagnetic high‐spin state. The factors influencing the equilibrium between high‐ and low‐spin Co3+ions are discussed.New compounds are presented in which the change of the equilibrium distribution among these two states is measurable because the energy difference between the two states is small. It is then possible to change the equilibrium by varying the temperature. The temperature dependence of the magnetic susceptibility of such a compound is anomalous as has been derived by Ballhausen and Liehr.