Quantum chemical calculations using the density functional theory (DFT) and some semi-empirical methods were performed on four sulphonamides (sulfaguanidine, sulfamethazine, sulfamethoxazole and sulfadiazine) used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between molecular structure and their inhibition efficiencies. The results of the quantum chemical calculations and experimental %IE were subjected to correlation analysis and indicate that their inhibition effect are closely related to E-HOMO, E-LUMO, hardness, polarizability, dipole moment and charges. The %IE increased with increase in the E-HOMO and decrease in E-HOMO - E-LUMO. The negative sign of the E-HOMO values and other kinetic and thermodynamic parameters indicates that the data obtained support physical adsorption mechanism. (c) 2008 Elsevier Ltd. All rights reserved.