Density functional theory (DFT) at the B3LYP/6-31G (d,p) and BP86/CEP-31G* basis set levels and ab initio calculations using the RHF/6-31G (d,p) methods were performed on four sulfonamides (namely sulfaacetamide (SAM), sulfapyridine (SPY), sulfamerazine (SMR), and sulfathiazole (STI)) used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between molecular structure and their inhibition efficiencies (%IE). The order of inhibition efficiency obtained was SMR > SPY > STI > SAM which corresponded with the order of most of the calculated quantum chemical parameters namely E-HOMO (highest occupied molecular orbital energy), E-LUMO (lowest unoccupied molecular orbital energy), the energy gap (Delta E), the Mulliken charges on the C, O, N, S atoms, hardness (eta), softness (S), polarizability (alpha), dipole moment (mu), total energy change (Delta E-T), electrophilicity (omega), electron affinity (A), ionization potential (I), the absolute electronegativity (chi), and the fraction of electrons transferred (Delta N). Quantitative structure activity relationship (QSAR) approach has been used and a correlation of the composite index of some of the quantum chemical parameters was performed to characterize the inhibition performance of the sulfonamides studied. The results showed that the %IE of the sulfonamides was closely related to some of the quantum chemical parameters but with varying degrees/order. The calculated %IE of the sulfonamides studied was found to be close to their experimental corrosion inhibition efficiencies. The experimental data obtained fits the Langmuir adsorption isotherm. The negative sign of the E-HOMO values and other thermodynamic parameters obtained indicates that the data obtained supports physical adsorption mechanism. (C)2009 Wiley Periodicals, Inc. Int J Quantum Chem 110:2614-2636,2010