The corrosion inhibition of steel in water from Western Desert (Egypt) crude oil by nine S-alkyl isothioronium halides was studied using open-circuit potential (OCP) measurements and potentiodynamic polarization tests. The objectives were to determine (a) the effect of chain length of the alkyl groups on the inhibition efficiency, (b) the effect of halide counter ion on the performance of the inhibitor, and (c) to apply a recent developed kinetic-thermodynamic model on the data and compare it with common adsorption isotherms. The number of active sites, binding constant and change of free energy of adsorption were computed for all inhibitors studied. The inhibition efficiency was correlated with the molecular structure of the inhibitors. It was found that each organic molecule replaces more than one adsorbed water molecule from the steel surface. Potentiodynamic polarization curves indicated that the compounds acted as mixed-type inhibitors. The OCP measurements showed that adsorption of S-alkyl isothioronium compounds at the steel surface is through two adsorbed layers.