Abstract: The lactonization of 2'-hydroxybiphenyl-2-carboxylic acid to 3, 4-benzocoumarin has been studied in aqueous solution, 30 OC, in the range of pH 0-6.5. As with simpler coumarinic acids, the rate-determining step changes from the breakdown of tetrahedral intermediates in the neutral or weakly acidic pH range to formation of intermediates below pH 2. In contrast, the lactonization of methyl-substituted dihydrocoumarinic acids shows ...