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Phenolic preparations from Vitis vinifera L. grape seeds are products commonly used in the formulation of dietary antioxidant supplements. In this article, we used a methodology (the oxygen radical absorbance capacity, ORAC) to evaluate the in vitro antioxidant capacity of commercial dietary grape seed products and studied the relationship of the antioxidant capacity with the phenolic composition of these products. The ORAC value of the different brands of commercial products studied (n = 16) varied from 2.71 to 26.4 micromol Trolox equivalents/mg (approximately equal to 10-fold difference). For four of these products, the batch-to-batch ORAC variation, expressed as a coefficient of variation of the mean, was 10.5% (n = 6), 13.1% (n = 3), 19.4% (n = 4), and 7.8% (n = 4). Analysis of monomeric and oligomeric flavan-3-ols by liquid chromatography-diode array detection (LC-DAD)/electrospray-mass spectrometry and procyanidins by thiolysis-LC-DAD also revealed large differences among the commercial grape seed products. Moreover, the ORAC value could be fitted to a regression model using variables from contents of individual phenolic compounds and procyanidins. The product-to-product and batch-to-batch variation in ORAC values and flavan-3-ol composition found among the commercial products studied demonstrated that they are poorly standardized, resulting in inconsistent composition and biological activity. [ABSTRACT FROM AUTHOR]