OH addition reactions of cationic,neutral and anionic forms of three sulfonamides (sulfamethoxazole,sulfadiazine and sulfapyridine) in aqueous solution were theoretically studied using density functional theory (DFT) method at the M06-2X/6-31 l+G(3df2p) level.Transition state theory was applied to estimate the secondary rate constants for these elementary reactions.The obtained results indicate that OH addition reactions of sulfonamides can take place spontaneously at standard conditions.The anionic form of three sulfonamides has the highest addition activity,while the corresponding cationic form is the most inactive addition reagent.The benzene ring of neutral forms of three sulfonamides is always a more favorable site for OH radical addition than the oxazole,pyrimidine or pyridine ring.C(3) or (and) C(5) atoms of benzene ring are the most favorable positions for OH addition occurring in benzene ring.Although the water solvent has no remarkable effect on OH addition reactions of neutral sulfonamides,it exerts a significant adverse influence on OH addition reactions of ionic sulfonamides.