Structures of a-hydroxybenzylic cations and their conjugate bases, which cover a wide variation of stability, were optimized by means of ab initio molecular orbital method at the RHF/6-31G* level. Total energies were calculated at the MP2/6-31G*//RHF/6-31G* ZPE (scaled 0.9) level. Calculated relative proton affinities of the respective neutral molecules (benzoyl compounds; conjugate bases of a-hydroxybenzylic cations) agreed well with the corresponding basicities in the gas phase. The geometries of a-amino-a-hydroxybenzyl and a-hydroxy-adimethylaminobenzyl cations were also optimized at the fixed dihedral angles between the cationic 2pp orbital and the benzene p orbital (f), and between the cationic 2pp orbital and lone pair electron orbital of the a-substituent (). The changes in Wiberg bond orders and the rotational potentials about f and showed that the degree of resonance interaction between the cationic center and phenyl ring is balanced by the electronic effects of a-substituents in benzylic cations. The obtained theoretical indices of all parent cations such as Mulliken population, Wiberg bond order and bond lengths were correlated linearly with the resonance demand parameter (r value) which were given by the Yukawa-Tsuno substituent effect analysis in the gas phase and in aqueous solution. These relationships are consistent with those for other benzylic cations such as destabilized carbocations and sterically hindered cations studied previously. This confirms that the empirical r value has a definitive physical meaning, i.e. a measure of the resonance interaction between the cationic center and the aryl moiety.