The extraction and chemical characterization of essential oils from Syzygium aromaticum L., Origanum vulgare L., Mentha spicata L. and Eremanthus erythropappus M. is reported, as well as an evaluation of their antioxidant activity and of their major constituents eugenol, carvacrol, carvone and α-bisabolol, respectively, using colorimetric, electroanalytical and theoretical methods. Essential oils were obtained by hydrodistillation, identified by GC/MS and quantified using a GC coupled to a flame ionization detector. The colorimetric evaluation of the antioxidant activity was based on stabilization of DPPH radicals, β-carotene bleaching, deoxyribose degradation and reducing power. The electrochemical behavior was studied using cyclic voltammetry, and the dissociation enthalpy of the OH bond was theoretically estimated at the density functional theory ωB97X-D/6-31+g(d,p) level. The essential oils from S. aromaticum and O. vulgare L., and their major constituents eugenol and carvacrol, yielded antioxidant responses similar to the synthetic ascorbic acid and BHT, while eugenol and the essential oil from S. aromaticum exhibited better performance than carvacrol and the essential oil from O. vulgare L. Overall, eugenol was found to be a promising compound to replace marketed antioxidants or at least to be used in association with them.