Antioxidant capacity for a wide range of natural or synthetic polyphenolics was comprehensively evaluated by vitamin C equivalent antioxidant capacity (VCEAC) assay using free blue/green ABTS radicals. The polyphenolics tested are grouped into the following categories: vitamins (beta-carotene, alpha-tocopherol, vitamin A, and vitamin C), phenolic acids (benzoic acid, phenylacetic acid, cinnamic acid, and their derivatives), flavonoids (anthocyanidin, flavanol, chalcone, flavanone, flavone, flavonol, isoflavone, and their derivatives), synthetic food additives (BHA, BHT, TBHQ, and PG), and other miscellaneous polyphenolics (ellagic acid, sesamol, eugenol, thymol, etc.). A positive linear relationship between VCEAC and the number of free OH groups around the flavonoid framework was found, whereas, for phenolic acids, the linear relationship was not as good as with the flavonoid aglycones. Groups of chemicals having comparable structures generally showed similar trends. Polyphenolics commonly showed a higher VCEAC compared to monophenolics. Compounds like gallic acid with 3 vicinal hydroxy substitutions on the aromatic ring in phenolic acids or like epigallocatechin with 3 vicinal hydroxy substitutions on the B ring in flavonoids showed the highest antioxidant capcity among the groups. In the flavonoids, 2 characteristic chemical structures were very important, the catechol moiety in the B ring and the 3-OH functional group in a chroman ring. Glycosylated flavonoids showed less potent antioxidant capacity than their aglycone alone. Synthetic antioxidant food additives (BHA, TBHQ, and BHT) conventionally used in the food industry were less effective antioxidants than ascorbic acid. Other naturally occurring polyphenolics tested followed the expected general trends of phenolic acids and flavonoids.