The corrosion and inhibitor adsorption processes in mild steel/1-methyl-4[4'(-X)-styryl] pyridinium iodides (X: -H, -CH3 and -OCH3)/hydrochloric acid systems was studied at different temperatures (25-60 degrees C) by means of hydrogen evolution (HE) and weight loss (WL) measurements. It was found that the studied compounds exhibit a very good performance as inhibitors for mild steel corrosion in 1.5 M HCl. Results show that the inhibition efficiency increases with decreasing temperature and increasing concentration of inhibitors. Good agreement between the results obtained from hydrogen evolution and weight loss measurements was appeared. It has been determined that the adsorption for the studied inhibitors on mild steel complies with the Langmuir adsorption isotherm at all studied temperatures. The kinetic and thermodynamic parameters for mild steel corrosion and inhibitor adsorption, respectively, were determined and discussed. On the bases of thermodynamic adsorption parameters, comprehensive adsorption (physisorption and chemisorption) for the studied inhibitors on mild steel surface was suggested. A good correlation between the substituent type and the inhibition efficiency of inhibitors through the application of Hammet relationship was obtained. Results show that with increasing the donor property of the substituent, the inhibition efficiency of the inhibitor is increased in the order: I-H < II-CH3 < III-OCH3. (C) 2008 Elsevier B.V. All rights reserved.