A theoretical evaluation of the g factor for Fe(III), Mn(II) and Cr(I) ions in MgO is done, taking into account localized states of the conduction band in the perturbation calculation. It is shown that the influence of the conduction band can be as important as that of the valence band. It is the competition of the influences of the two bands which explains the sign inversion of (g - 2) between Fe(III) and Cr(I).