The aromatic13C substituent constant for the -CH2COOCH3group is obtained by selectively irradiating phenylacetic acid methyl ester. The value so obtained is used for refining the literature values of the -OH and -OCH3substituent constants and for assigning all the aromatic lines in six derivative isomers. Formetaandparaisomers, calculated and experimental values for all the aromatic13C resonances are within 0.2 ppm. For purposes of evaluating a rapid qualitative analytical method for the biologically important phenylacetic acid derivatives, the1H resonances are obtained and assigned, and the benzylic methylene chemical shifts are shown to be specifically diagnostic of aromatic isomerism for these derivatives.