The accuracy of the measurement of aspirin by high-performance liquid chromatography (HPLC) is reduced by its hydrolysis into salicylic and acetic acids during sample preparation. The ready and quantitative transacetylation of 3-aminophenol by aspirin, giving 3-hydroxyacetanilide (which is stable), has been utilized as a pre-column reaction for the determination of aspirin either alone or in the presence of acetaminophen and caffeine by reversed-phase HPLC.