[en] The cross section for resonant charge transfer in ion-atom collisions is determined by the difference in energies of the gerade and ungerade states of the quasimolecule formed during the collision. Calculations of this energy difference were performed for many atoms, taking account of the effects of spin-orbit interactions and the indistinguishability of the electrons. Results for this energy difference were used in the numerical calculations of charge transfer cross sections for all the alkali positive ions and the negative ions of sodium and caesium. Positive ions of krypton, xenon, calcium and uranium were also studied. The calculations covered the energy range from 0.025 eV to 1000 eV. Whenever possible, comparisons with experimental work and other theories were made. In most, but not all, cases the results are consistent with the experiments. It is found that the effect of the polarisation interaction is negligible for ions of energies higher than 25 eV; at thermal energies, however, it leads to considerably larger cross sections. It is also found that the two state quasiadiabatic approximation is not applicable to ions of energies of about 1 keV or higher