In order to relate the corrosion inhibition effect of organic compounds for steel in acid media to their molecular characteristics, SCF calculations have been performed for thiourea and nine derivatives, for two further sulphur-containing organic compounds and for urea and two derivatives. The structure of all compounds has been optimised, ionisation potential, dipole moments and the electron density distribution has been computed. It is seen that the dominant molecular parameter for the inhibition effect is the ionisation potential and that there is a clear relation between the increase in corrosion inhibition and the decrease of the ionisation potential.