Grape skins and seeds, byproducts of the grape juice/wine industry can be utilized as dietary supplements or used in functional foods because of their potential health functional components. The effectiveness of different aqueous solvents in extracting phenolics from muscadine seed powder was investigated. Antioxidant capacities of byproduct grape seeds and skins from Vitis vinifera varieties Merlot and Chardonnay and the seeds of Vitis rotundifolia variety Muscadine were determined using oxygen radical absorbance capacity (ORAC) assay. The contribution of major phenolics in these byproducts to the total antioxidant capacities was also evaluated. Finally, the stability of muscadine seed extract (MSE) was determined in a puffed rice cereal bar during storage at different temperatures. Aqueous solutions containing 60% ethanol (190 proof), 60 to 70% methanol, and 50 to 75% acetone were better than any single compound solvent system in extracting phenolics from muscadine grape seed powder. Antioxidant capacities of Chardonnay, Merlot and Muscadine grape seed powders were 637.8, 344.8 and 310.8mol TE/g d.m., respectively. Gallic acid, catechin and epicatechin concentrations on a dry basis were 68, 7, and 69mg/100g in Muscadine seeds, 10, 211, and 303mg/100g in Chardonnay seeds, and 7, 74, and 83mg/100g in Merlot seeds, respectively. Concentrations of these three compounds were lower in winery byproduct grape skins than seeds. These three major phenolic constituents of grape seeds contributed less than 17% to the antioxidant capacity measured as ORAC. Phenolic constituents of MSE in a puffed rice cereal bar were more stable at 19C compared to 37C over three months. Antioxidant capacities of the food product supplemented with the MSE measured as ORAC was reduced over time with no difference attributed to storage temperature (p>0.05). Moreover, MSE provided natural antioxidant activity by inhibiting lipid peroxidation. Byproducts of grape juice/wine industry contain valuable phenolic antioxidant compounds. Procyanidins other than monomers are responsible for most of the superior antioxidant capacity of grape seeds. Health conscious consumers can have the health benefits of cereal products supplemented with MSE.