The relative gas-phase acidities of 25 ring-substituted benzoic and phenylacetic acids were theoretically determined with proton-transfer reactions. The energies and geometries of the acids and their corresponding anions, which were involved in the reactions, were calculated at the B3LYP/6-311+G(2d,p) level of theory. The obtained substituent effects were compared with each other and those of phenols. The acidities of the benzoic and phenylacetic acids were governed by three kinds of electronic effects (inductive, resonance, and saturation effects), which confirmed that the acidities were reflected in the nature of the benzoate and phenylacetate anions, respectively. Substituent effect analyses with an extended Yukawa–Tsuno equation, , gave excellent linear correlations for both anionic systems. The degrees of through-resonance and saturation effects in the phenylacetate anion, as reflected by the resultant r− and s values, were unexpectedly larger than those in the benzoate anion, which were mainly attributed to hyperconjugation and through-space interactions between the anionic moiety and the benzene π-electron system, respectively. The acidities of the benzoic acids (or stabilities of the benzoate anions) constituted a better system than the acidities of phenylacetic acids (or stabilities of the phenylacetate anions) for a standard of the normal substituent constants (σ0) for anions in the gas phase, in contrast to solution-phase results.