The pi-donating effects of pi-accepting X-substituents in substituted benzylic cations, X-C6H5-CHR+ where R = CF3, H and OCH3, and X = p-NH2, p-OCH3, p-CH3, H, p-F, p-Cl, p-CHO, m-CN, p-CN, m-NO2 or p-NO2, have been studied theoretically by using isodesmic hydride transfer reactions at various levels of theory. It might be difficult to determine the pi-donating effects of pi-acceptors using the simple Hammett-type linear equation, because it is not sensitive enough to include small pi-donating effects. Therefore, this effect was estimated using the NBO deletion energy (Delta E-D) of the second-order charge-transfer interaction (Delta E-ct) between the pi-orbitals (or lone pair orbitals) of the X-substituent and the pi*-orbitals of phenyl ring. The extents of pi-donating effects increased in the order X = p-NO2 < p-CHO < p-CN << p-Cl for both neutral and cationic species, and these effects were found to be more important for para- than for meta-substituents. Moreover, this could represent a general trend for pi-donation by pi-acceptors. On the other hand, the effects of R-substituents on this T-donating effect were found to be in the order R = OCH3 < H congruent to CF3, as predicted by natural resonance theory (NRT) analyses.