Sub-additivity of antioxidant activities in assays of total antioxidant capacity has been reported previously and ascribed to binding of low-molecular weight antioxidants such as flavonoids by proteins. We demonstrate that this phenomenon is much more common and concerns also interactions between typical low-molecular weight oxidants in the assays of ABTS- decolorization and protection of fluorescein from AAPH-induced bleaching. The subadditive interactions between antioxidants may affect quantitative considerations drawn from in vitro assays of antioxidant capacity of biological samples.