The salicylic acid oxidation has been explained by a mechanism involving single electron transfer oxidation and single hydrogen transfer using quantum chemistry calculations at the B3LYP theory level, together with the 6-311G** basis set. These methods were employed to obtain energy (E), ionization potential (IP), bond dissociation energies (BDE), and spin density distribution of the salicylic acid. The results show no discrepant behaviors between electron and hydrogen transfer in the regioselective hydroxylation of salicylic acid by cytochrome P-450. The unpaired electron remains localized on the O 7 phenolic oxygen (0.26 and 0.38), C 1 carbon atoms at the carbonyl group (0.12 and 0.28), C 2 carbon atom at the hydroxyl group (0.15 and 0.00), C 3 carbon atom at the hydroxyl group (0.22 and 0.30), and C 5 carbon atom (0.40 and 0.41) for cation free radical and semiquinone form, respectively.Calculations of spin densities showed that chemistry reactivity is more favored in the positions C 5 [ C 3 [ C 1 to form salicylate derivatives. These results supported the salicylic acid as scavenger derivatives in the lipid peroxidation. Furthermore, we suggest a conventional proton and secondary electron abstraction, and semiquinone form by [1,5] hydrogen shift between phenol and carbonyl groups.