Mechanistic details of selective conversion of o-methoxyphenols to the corresponding catechols using a Cu²⁺–ascorbic acid–O₂ system, were studied. 2,5-Dimethoxyphenol was converted predominantly to the o-demethylated compound and partially to the m-demethylated one. Anisole with no phenolic hydroxy group was much less reactive. When guaiacol (1), [Me-²H³]guaiacol and 2-[²H₃]methoxy-6-methoxyphenol were used as substrates, moderate intermolecular and intramolecular kinetic isotope effects were observed (1.4–1.9). [¹⁸O]Catechol was derived from 1 in nine-fold excess over [¹⁶O]catechol when the reaction was run in an ¹⁸O₂ atmosphere with natural water as a solvent, though no [¹⁸O]catechol was formed when using natural O₂ and H₂¹⁸O. It was determined that the Cu²⁺–ascorbic acid–O₂ system operates in a monooxygenase mode because the oxygen atom of dioxygen (not water) was incorporated into the products, and this oxidative conversion proceeded mainly via ipso-substitution at the methoxy position, probably with hydroxyl radical coordinated to the cupric ion as the active oxygen species.