The conformational behavior, molecular geometry and electronic structure of quercetin were investigated using the semiempirical AM1 and PM3 methods. Results reveal that quercetin has a nonplanar molecular structure, with cross-conjugation occurring at the C ring. Calculations were also performed for quercetin radical species at the OH groups, showing the presence of three radicals in a narrow range of energy. An interpretation of the antioxidative process mechanism, exerted by quercetin as a free radical scavenger, relies on two isoenergetic radicals with extended electronic delocalization between adjacent rings, also having cross-conjugated systems and being affected by the experimental environment influencing their relative order.