A simple but efficient method for determination of labile protons in drug metabolites using post-column infusion of deuterium oxide (D2O) in liquid chromatography/mass spectrometry (LC/MS) experiments with electrospray ionization and time-of-flight mass spectrometry is described. The number of exchangeable protons in analytes, i.e. hydroxyl, amine, thiol and carboxylic acid protons, can easily be determined by comparing the increase in m/z values after H/D-exchange occurring online between a HPLC column and electrospray ion source. Especially, the hydroxyl metabolites and S/N-oxides with the same accurate mass can be distinguished. A good degree of exchange was obtained in repeatable experiments. Only a low consumption of D2O is needed in a very easy and rapidly set-up procedure. The method is applied in the study of metabolites of omeprazole and imipramine in human and mouse in vitro samples, respectively, together with exact mass data obtained from time-of-flight mass spectrometric experiments.