The corrosion inhibition efficiencies of some triazole, oxadiazole and thiadiazole derivatives for steel in presence of acidic medium have been studied by using AM1, PM3. MINDO/3 and MNDO semi-empirical SCF molecular orbital methods. Geometric structures, total negative charge on the molecule (TNC), highest occupied molecular energy level (E-HOMO), lowest unoccupied molecular energy level (E-LUMO), core-core repulsion (CCR), dipole moment (mu) and linear solvation energy terms, molecular volume (V-i) and dipolar-polarization (pi(*)), were correlated to corrosion inhibition efficiency. Four equations were proposed to calculate corrosion inhibition efficiency. The agreement with the experimental data was found to be satisfactory: the standard deviations between the calculated and experimental results ranged between +/- 0.03 and +/- 4.18. The inhibition efficiency was closely related to orbital energies (E-HOMO and E-LUMO) and mu. The correlation between quantum parameters and experimental inhibition efficiency has been validated by single point calculations for the semi-empirical AM I structures using B3LYP/6-31G** as a higher level of theory. The proposed equations were applied to predict the corrosion inhibition efficiency of some related structures to select molecules of possible activity from a presumable library of compounds. (c) 2005 Elsevier Ltd. All rights reserved.