Density functional theory B3LYP and Hartree-Fock methods with 6-311??G** basis set were utilized to study the relationship between electronic structure and corrosion inhibition efficiencies of protonated and non-protonated forms of 3-formyl 8-hydroxy quinoline and 5-naphthylazo-8-hydroxyquinoline (5NA8HQ) in acidic media in gas and solvent phase. Quantum chemical parameters including highest molecular orbital energy, lowest unoccupied molecular orbital energy, energy gap (DE g ), the fraction of electrons transferred (DN), and energy change during charge transfer (DE) were calculated. Protonation energy calculations showed that the favorite protonation site of 5NA8HQ is the N12 position, which is confirmed by the result of previously reported experimental investigations. Calculations on the inhibitor/iron system were performed using the B3LYP/6-311??G** method and found that azo nitrogens of 5NA8HQ have better interaction with iron. Also, nuclear quadrupole resonance parameters indicate that azo nitrogens (N11 and N12) are the best sites for interaction with iron.