The lipophilic antioxidant vitamin E was used as a surface modifier (or coating agent) of hollow-fiber hemodialyzer membranes with the aim of increasing their biocompatibility and preventing oxidative stress, which are the main clinical drawbacks in hemodialysis (HD) therapy. At present, the redox chemistry of vitamin E-modified dialyzers is not well characterized and there is no standard method to assess the antioxidant capacity of these biomembranes under conditions that simulate those observed during HD therapy. With this study, we developed an original online method to determine the antioxidant capacity of vitamin E-modified dialyzer membranes during circulation experiments. This method is based on a spectrophotometric assay known as the ferric reducing/antioxidant power assay (FRAP). The principle of FRAP and its application to the qualitative and quantitative assessment of miniaturized polysulfone (PS)-based vitamin E-modified dialyzers (PS-VE) were verified by the accurate in vitro analysis of the iron-catalyzed oxidation of vitamin E. The antioxidant capacity of miniaturized PS-VE samples assessed in this study was of 14.5 microM Fe(2+), which corresponded to the transformation of nearly one-third of the vitamin E bound to the hollow-fiber membrane to its oxidation end product alpha-tocopherol quinone. This method shows good reproducibility and intra- and inter-assay precision, and can be easily adapted to determine the redox activity of every type of vitamin E-modified dialyzers during technological investigation, manufacturing control and clinical research.