The reaction of verapamil with concentrated sulfuric acid was studied. It was found, in disagreement with a published report, that the reaction did not hydrolyze the nitrile group of the drug to the corresponding carboxylic acid, but, instead, produced a sulfonated derivative. Analysis of the product using several spectroscopic techniques indicated that the sulfonic acid moiety was introduced into the verapamil molecule at the 6-position of the 3,4-dimethoxyphenyl ring distant to the chiral center.