The effective bond charge q e f f , as determined from experimental values of the third-order optical susceptibility in alkali halides, is analyzed. The correlations of q e f f with the ratio of the cation to anion radius, lattice constant and Szigeti effective charge are taken into account. The relationships between q e f f and other parameters describing the ionicity show that the magnitude of q e f f can be considered as one more parameter related to the ionicity of the crystals.