On the basis of Density Functional Theory (DFT) calculations it was shown in our previous work [1] that namely the 3-OH and 4-OH hydroxyl groups of caffeic acid (CA) are responsible for the synergistic antioxidant effect with glutathione taking place in their joint reaction with the cation-radical of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS*+). At the same time the experimental data (see [2]) indicate on the existence of the synergistic effect between CA and quercetin (Q) in their joint FRAP test. Thus, it seems to us interesting to investigate by means of the DFT method the structural features of CA and Q which could cause the possible antioxidant synergistic effect in their joint reaction with ABTS*+. The data of our DFT calculations show that in the case when neutral molecules of CA and Q form their complexes with ABTS*+ (by means of one of the two –SO3H groups of the cation-radical ABTS*+) the electronic density of the unpaired electron of ABTS*+ remains mainly on the ABTS*+’s structural fragments of these complexes. However, in their turn, these complexes: [CA & ABTS*+] and [Q & ABTS*+] can easily interact respectively with the anions Q- and CA- arising due to deprotonation of the 4-OH group of Q and 4-OH group of CA. The given interactions lead to the situation, when the ABTS*+’s structural fragments of both the above complexes do not contain any unpaired electrons. Here it is to be noted, that 3-OH’ and 3-OH groups of Q and CA also take part in their co-ordination with –SO3H groups in the corresponding complexes [CA & ABTS*+] and [Q & ABTS*+] (Figure1). Figure 1. The interaction of the 3-OH’ and 4-OH’ hydroxyl groups of quercetin with the SO3H group of ABTS*+. Thus, basing on the DFT calculations, one can conclude that in the case under study the antioxidant synergistic effect in the joint reaction of CA and Q with ABTS*+ will take place. Moreover, namely 3-OH and 4-OH hydroxyl groups of CA along with 3-OH’ and 4-OH’ hydroxyl groups of Q will be responsible for the arising of the given effect.