This work deals with the chlorine dioxide (ClO(2)) reactivity with three representative beta-lactam antibiotics (penicillin, amoxicillin and cefadroxil) that can be present in natural aquatic resources. Due to the wide use of ClO(2) as disinfection agent our work is of interest to determine the fate of these antibiotics during the water treatment process. Our study shows that antibiotics react stoichiometrically with ClO(2) because increasing amounts of ClO(2) lead to increasing antibiotic disappearance. Concerning the influence of antibiotic structure, penicillin reacts sluggishly with ClO(2), whereas amoxicillin and cefadroxil are highly reactive at either neutral or basic pH. For both reactive antibiotics, hydroquinone together with a wide range of 4-substituted phenols were detected as products. Pretreatment with ClO(2) before chlorination of aqueous solutions of antibiotics reduces the trihalomethane formation as compared with analogous chlorination without ClO(2) pretreatment.