Two selectively O-18-labeled phthalides containing O-18 either in the carbonyl (i.e., C=O-18) or in the alkoxy group (i.e., O=C-O-18) of the lactone ring were prepared in good yields (ca. 70%) from the corresponding methyl 2-(bromomethyl)benzoate and its selectively C=O-18 labeled derivative, respectively, using water-assisted cyclization. The reaction mechanism involving formation of a cyclic carboxonium bromide and its water-assisted decomposition to the lactone is proposed. (c) 2013 Elsevier Ltd. All rights reserved.