The substituent effects on the relative gas-phase stabilities of 16 kinds of benzylic anion systems were determined theoretically using proton transfer reactions. The energies and geometries of ring-substituted anions and corresponding neutral species, which are involved in the reactions, were calculated at the B3LYP/6-311+G(2d,p) level of theory. The obtained substituent effects were compared with one another to reveal that the gas-phase stabilities of benzylic anions are governed by an inductive effect and two additive effects. By considering the geometrical changes of the anions, these two effects were assigned to resonance and saturation effects, which stabilize anions by para +R groups and electron-releasing (ER) groups, respectively. The saturation effect was found for not only para but also meta substituents and functions through both π and σ bonds. Statistical analyses of the substituent effects ascertained that the behavior of the substituent effects can be quantitatively described by three terms – that of the inductive effect, the resonance effect, and the saturation effect – using an extended Yukawa–Tsuno equation, . Copyright © 2010 John Wiley & Sons, Ltd.