The mechanism of electrochemical oxidation of catechol (1a), 3-methylcatechol (1b) and 3-methoxycatechol (1c) in the presence of benzenesulfinic acid (3) as a nucleophile has been studied in an aqueous solution using cyclic voltammetry and controlled-potential coulometry. The results indicate that the catechol derivatives (1a-1c) are converted to sulfone derivatives (4a-4c) through Michael addition of benzenesulfinate to anodically generated o-quinones (2a-2c). The electrochemical synthesis of 4a-4c has been successfully performed in an undivided cell in good yields and purity.