Antioxidant mechanisms of curcumin, bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, have been studied by laser flash photolysis and pulse radiolysis. The keto−enol−enolate equilibrium of the heptadienone moiety of curcumin determines its physicochemical and antioxidant properties. In neutral and acidic aqueous solutions (from pH 3 to 7), the keto form dominates, and curcumin acts as an extraordinarily potent H-atom donor. The reaction rate constant with the methyl radical (3.5 ± 0.3) × 109 M-1 s-1 is close to diffusion control in 40% aqueous DMSO at pH 5. The tert-butoxyl radical reacts with curcumin in acetonitrile solutions at a diffusion controlled rate, k = (7.5 ± 0.8) × 109 M-1 s-1. The apparent site of reaction is the central CH2 group in the heptadienone link, which has two labile hydrogens. This is supported by comparing the reaction patterns of curcumin and dehydrozingerone (DHZ) (“half-curcumin”, 4-(4-hydroxy-3-methoxyphenyl)-3-buten-2-one). DHZ does not react with the methyl radical, indicating that the presence of the labile hydrogens is crucial for the H-atom donating ability of curcumin. The tert-butoxyl radical reacts with DHZ at almost an order of magnitude lower rate (1.1 ± 0.1) × 109 M-1 s-1, clearly abstracting an H-atom from the phenolic OH group. The reaction mechanism of curcumin changes dramatically above pH 8, where the enolate form of the heptadienone link predominates. As a consequence, the reaction of the methyl radical diminishes completely in alkaline media, and the phenolic part of the molecule takes over as (electron donor) reaction site. The electron donating ability of curcumin is assessed from the measurements of one-electron-transfer equilibria of DHZ radicals. Reduction potential of the DHZ phenoxyl radical, E(pH = 6.5) = 0.83 ± 0.06 V, and E(pH = 13.0) = 0.47 ± 0.06 V vs NHE, which may be expected for an ortho-methoxy-substituted phenoxyl radical, indicate only moderate electron-donating ability. The importance of H-atom donation vs electron donation in free radical scavenging and antioxidant mechanisms of curcumin is discussed.