We have performed Density Functional Theory B3LYP/6-311++G** calculations of reaction enthalpies of antioxidant action mechanisms for nine isoflavones. O–H bond dissociation enthalpies, ionization potentials, proton dissociation enthalpies, proton affinities and electron transfer enthalpies related to Hydrogen Atom Transfer (HAT), Single Electron Transfer-Proton Transfer (SET-PT) and Sequential Proton-Loss Electron-Transfer (SPLET) mechanisms were investigated in gas- and solution-phases. Studies on the radical scavenging ability of isoflavones, contrary to various flavonoids, are still scarce. Thus, understanding of its thermodynamics can be considered beneficial. The selection of isoflavones (daidzein, formononetin, genistein, biochanin A, prunetin, 6-hydroxydaidzein, glycitein, orobol and santal) enables us to evaluate the effects of various structural features, such as the presence of methoxy (4′-OMe, 6-OMe, 7-OMe) and hydroxy (3′-OH, 5-OH, 6-OH) groups, on studied reaction enthalpies. The obtained results show that HAT can be attributed predominantly to the B ring, while SPLET takes place preferentially in the A ring, as was also indicated in experimental works.