Weight loss measurement and electrochemical techniques were used as the methods for studying the inhibition effects of 4-methyl-2-(methylthio)-3-phenylthiazol-3-ium iodide (ST1) on carbon steel corrosion in 0.5 M H2SO4. The results reveal that corrosion rate of carbon steel decreases with addition of ST1 in 0.5 M H2SO4. The corrosion rate and inhibition efficiency were found to depend on the concentration of the inhibitor. Inhibition efficiencies up to 99% can be obtained. The polarization data indicated that the inhibitor was of mixed type, with predominant effect on the cathodic partial reaction. Changes in impedance parameters (charge transfer resistance, Rt, and double layer capacitance, Cdl) confirm that corrosion inhibition was due to adsorption on the metal surface following the Langmuir adsorption isotherm. The results obtained from the three different techniques were in good agreement. The influence of the immersion time shows that the inhibiting molecules form a stable and efficient film. XPS analyses clearly show that the adsorption mechanism of ST1 on the carbon steel surface in 0.5 M H2SO4 solution is mainly physical adsorption.