The phthaloyl chloride isomers, terephthaloyl chloride (TCl) and isophthaloyl chloride (ICl), are high production volume chemicals used in polymers to impartflame resistance, chemical resistance, and temperature stability and as water scavengers. In these studies, we determined the hydrolytic stability of TCl and ICl and their hydrolysis products in aqueous solutions. Hydrolysis rates for TCl and ICl were initially determined by gas chromatography/flame ionization detection in water buffered at pH 4.0, 7.0, and 9.0 and 0 degrees C for up to 30 min. Subsequent studies determined the products from TCl and ICl hydrolysis. The parent phthaloyl chlorides (TCl and ICl), their intermediate hydrolysis products (designated as the "half-acids"), and their stable hydrolysis products (terephthalic acid (TPA) and isophthalic acid (IPA)) were determined by high-performance liquid chromatography. The half-lives (t(1/2)) of TCl and ICl ranged from an average of 1.2 to 2.2 min and from 2.2 to 4.9 min, respectively, at pH 4-9 and 0 degrees C. The observed first-order rate constants (k(obs)) ranged from an average of 530 to 1100 (x 10(5) s(-1)) for TCl and 240 to 520 (x 10(5) s(-1)) for ICl. Both phthaloyl chlorides formed their respective short-lived intermediates, in which one of the two carboxylic acid chloride functionalities reacts with water to form the carboxylic acid ("half-acid"). Subsequently, the half-acids underwent further hydrolysis so that greater than 90% of the initial phthaloyl chloride hydrolyzed in less than 60 min at 0 degrees C. The hydrolysis products TPA and IPA were hydrolytically stable, undergoing no further transformations after 20 min at pH 7 and 25 degrees C. This work demonstrated that TCl, ICl, and their respective half-acids will not be persistent in aqueous systems for a time sufficient to have a sustained toxicological effect on aquatic organisms (less than 1 h). Performing additional aquatic toxicity studies, biodegradation studies, and potentially mammalian studies on TCl and ICl are unnecessary since the existing information on TPA and IPA with the hydrolysis data presented here is sufficient to address questions on the fate and effects of these two substances in aqueous environments.