The geometries, stabilities, and antioxidant activities of L-Ascorbic acid (1a), D-erythroascorbate (2a), and D-erythroascorbate glucoside (3a) as well as their sulfur and selenium derivatives are systematically investigated by using density functional theory. Emphasis is placed on studies of the two main mechanisms, that is, hydrogen atom donation and single-electron transfer, and the O—H bond dissociation enthalpy and the ionization potential are computed in the gas phase and water solution. The calculated results indicate that the 2-OH group in the five-membered ring acts as an important H atom donor to free radicals. The 2-OH radical spin density distribution shows that the unpaired electron is mostly located at the C3 atom of the five-membered ring and partially at the vicinal O atoms, proving that a certain delocalization of the odd electron is effective in the five-membered ring. In water aqueous solution, the antioxidant capacity and the electron donating ability are increased as the O atom in the five-membered ring of 1a, 2a, and 3a is replaced by S and Se, respectively, in good agreement with experimental measurements; Furthermore, their antioxidant capacities are enhanced as compared with the standard antioxidant (resveratrol). © 2013 Wiley Periodicals, Inc.