The spin hamiltonian theory is generalized so as to allow to introduce into it the charge transfer states. The contribution of these additional terms is numerically estimated in the case of ions of 3 d³ (Cr³⁺, Mn⁴⁺) and 3 d⁸ (Ni²⁺) configurations.

In the case of the Cr³⁺ and Mn⁴⁺ ions, it is shown that, by neglecting the influence of the charge transfer levels, the degree of covalency calculated from the results of the paramagnetic resonance may be overestimated by a factor as high as two. If one takes account of the overlap integrals, this effect is still more marked.

On the contrary, the charge transfer states contribute a completely negligible part in the case of the Ni²⁺ ion.