The effects of thiourea (TU), methylthiourea (MTU) and phenylthiourea (PTU) on the corrosion behaviour of mild steel in 0.1 M solution of H2SO4 have been investigated in relation to the concentration of thioamides (-CS-NH2). These compounds are all similar on one side of the thiocarbonly group and different on the other. The experimental data obtained using the techniques of R-p (polarisation resistance) and EIS (electrochemical impedance spectroscopy). The results showed that these compounds revaled a good corrosion inhibition, phenylthiourea being the most efficient and thiourea the least. The effect of molecular structure on the inhibition efficiency on corrosion has been investigated by ab initio quantum chemical calculations. The electronic properties such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, LUMO-HOMO energy gap and molecular orbital densities were calculated. The relations between the inhibition efficiency and some quantum parameters have been discussed and obvious correlations were found. The highest values of the HOMO densities were found in the vicinity of the sulphur atom indicating it as most probable adsorption centre. (C) 2004 Elsevier B.V. All rights reserved.