The density functional theory (DFT) at the B3LYP/6-31G (d,p) and B3LYP/6-311G(d,p) basis set levels and ab initio calculations using the HF/6-31G (d,p) and HF/6-311G(d,p) methods were performed on four rhodanine azosulpha drugs (namely 5-sulfadiazineazo-3-phenyl-2-thioxo-4-thiazolidinone, 5- sulfametllazineazo-3-phenyl-2-thioxo-4-thiazolidinone, 5-sulfadimethoxineazo-3-phenyl-2-thioxo- 4-thiazolidinone, and 5-sufamethoxazoleazo-3-phenyl-2-thioxo-4-thiazolidinone) used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between the molecular structure of the rhodanine azosulpha drugs and inhibition efficiency(%IE). The quantum chemical parameters/descriptors, namely, E-HOMO (highest occupied molecular orbital energy), E-LUMO (lowest unoccupied molecular orbital energy), the energy difference (Delta E) between E-HOMO and E-LUMO, dipole moment (mu), electron affinity (A), ionization potential (I), the absolute electronegativity (X), absolute hardness (eta), softness (sigma), polarizability (alpha), the Mulliken charges, and the fraction of electrons (Delta N) transfer from inhibitors to iron, were calculated and correlated with the experimental %IE. Quantitative structure activity relationship (QSAR) approach has been used, and a composite index of some quantum chemical parameters/descriptors was performed to characterize the inhibition performance of the studied molecules. The results showed that the inhibition efficiency (%IE) of the rhodanine azo sulfa drugs studied was closely related to some of the quantum chemical parameters/descriptors but with varying degrees of correlation coefficient (R-2). The %IE also increased with the increase in E-HOMO and decrease in E-HOMO-E-LUMO; and the areas containing N atoms are the most possible sites for bonding to the metal iron surface by donating electrons to the metal. The HOMO orbitals consist of 61.73-63.04% double bonded S atom (7(S)), and most of the rest are concentrated on the rhodanine group; so, the rhodanine molecule plays an important role in bonding of sulfa drugs with metal atom in the corrosion process. The calculated/estimated %IE of the compounds studied was found to be close to the experimental %IE. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 1003-1018, 2010.