[reaction: see structure] The antioxidant activity of curcumin (1, 7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) was determined by inhibition of controlled initiation of styrene oxidation. Synthetic nonphenolic curcuminoids exhibited no antioxidant activity; therefore, curcumin is a classical phenolic chain-breaking antioxidant, donating H atoms from the phenolic groups not the CH(2) group as has been suggested (Jovanovic et al. J. Am. Chem. Soc. 1999, 121, 9677). The antioxidant activities of o-methoxyphenols are decreased in hydrogen bond accepting media.