In antioxidant competition assays, an antioxidant (A) and a detector compound (D) compete for a reactive species (R). In the evaluation of these assays, it is tacitly assumed that all of R is captured by either D or A Due to the - by definition - high reactivity of R, unspecific reactions of R are likely to occur and neglecting these reactions will result in a systematic underestimation of antioxidant activity It was shown that in the standard hydroxyl radical scavenging assay this was indeed the case, the inaccurate mathematical evaluation resulted in ail underestimation of antioxidant activity of 25% in this competition assay The systematic underestimation of antioxidant activity can be prevented by using ail adjusted Stern-Volmer equation that takes into account that only part of R is captured by D or A (C) 2010 Elsevier Inc All rights reserved