The inhibition of corrosion of iron by thiourea (TU) and its N-substituted derivatives has been investigated using semiempirical molecular orbital theory. The high efficiency of TU with low concentration is attributed to strong adsorption of molecular species through the active center C-S bond. The protonation process appeared to be the effective parameter at high concentration with lowering of the inhibition efficiency of the TU. Alkyl N-substituted TU with the least protonation and with the maximum adsorption is a better inhibitor than TU and aryl N-substituted TU. The thiocarbohydrazide molecule shows the least efficiency of our investigated inhibitors with the highest protonation due to high charge density on the sulphur atom, lengthening and weakening of the C-S bond, and high reactivity with small energy separation between the frontier molecular orbitals. (C) 2004 Elsevier B.V. All rights reserved.