The copper(II) complexes 1H and 1Ar(X), supported by the N,N-di(2-pyridylmethyl)benzylamine tridentate ligand (LH) or its derivatives having m-substituted phenyl group at the 6-position of pyridine donor groups (LAr(X)), have been prepared, and their reactivity toward H2O2 has been examined in detail at low temperature. Both copper(II) complexes exhibited a novel reactivity in acetone, giving 2-hydroxy-2-hydroperoxypropane (HHPP) adducts 2H and 2 Ar(X), respectively. From 2Ar(X), an efficient aromatic ligand hydroxylation took place to give phenolate-copper(II) complexes 4 Ar(X). Detailed spectroscopic and kinetic analyses have revealed that the reaction proceeds via an electrophilic aromatic substitution mechanism involving copper(II)-carbocation intermediates 3Ar(X). Theoretical studies at the density functional theory (DFT) level have strongly implicated conjugate acid/base catalysis in the O-O bond cleavage and C-O bond formation steps that take the peroxo intermediate 2Ar(X) to the carbocation intermediate 3Ar(X). In contrast to the 2Ar(X) cases, the HHPP-adduct 2H reacted to give a copper(II)-acetate complex [Cu II(LH)(OAc)](ClO4) (6H), in which one of the oxygen atoms of the acetate co-ligand originated from H 2O2. In this case, a mechanism involving a Baeyer-Villiger type 1,2-methyl shift from the HHPP-adduct and subsequent ester hydrolysis has been proposed on the basis of DFT calculations; conjugate acid/base catalysis is implicated in the 1,2-methyl shift process as well. In propionitrile, both 1H and 1Ar(X) afforded simple copper(II)-hydroperoxo complexes LCuII-OOH in the reaction with H2O2, demonstrating the significant solvent effect on the reaction between copper(II) complexes and H2O2.