The linear dependence of the standard ion transfer potential Delta(O)(W) phi degrees of polyanions on their surface field strength E is elucidated on the basis of Mulliken’s charge-transfer (CT) complex theory. Quantum chemical considerations based on a simple model, where an electron in the 2p orbital (HOMO) of the surface oxygen atom of the polyanion partially transfers to the 4a(1) molecular orbital (LUMO) of water, have shown that the CT interaction energy per primary solvent is a function of E, approximately a quadratic equation. This leads to the apparent linear dependence of Delta(O)(W) phi degrees (exactly, the charge-dependent part) on E. The theoretically estimated coefficient of the linear term is in fair agreement with the experimental value, suggesting that the CT interaction plays the most important role in the linear dependence. In this paper, comments on Aoki’s theory of 1995 (J. Electroanal. Chem., 386 (1995) 17) are also offered.