The structures, energetics, vertical and adiabatic ionization potentials, electron affinities, and global reactivity descriptors of antioxidant vitamins (both water- and fat-soluble) in neutral, positively charged, and negatively charged states were investigated theoretically. We worked within the framework of first-principles density functional theory (DFT), using the B3LYP functional and both localized (6-311G+(d,p) and plane-wave basis sets. Solvent effects were modeled via the polarizable continuum model (PCM), using the integral equation formalism variant (IEFPCM). From the computed structural parameters, ionization potentials, electron affinities, and spin densities, we deduced that these vitamins prefer to lose electrons to neutral reactive oxygen species (·OH and ·OOH), making them good antioxidants. Conceptual DFT was used to determine global chemical reactivity parameters. The computed chemical hardnesses showed that these antioxidant vitamins are more reactive than neutral reactive oxygen species (ROS), thus supporting their antioxidant character towards these species. However, in the neutral state, these vitamins do not act as antioxidants for [Formula: see text]. The reactivity of vitamins towards ROS depends on the nature of the solvent. Amongst the ROS, ·OH has the greatest propensity to attract electrons from a generic donor. The reactivities of fat-soluble vitamins towards neutral reactive oxygen species were found to be larger than those of water-soluble vitamins towards these species, showing that the former are better antioxidants.