Acid solutions such as hydrochloric and sulfuric acids are used in acidic processes in petroleum industries for acid cleaning of boilers and acidizing of oil wells. To prevent the acid corrosion of metals, inhibitors should be used. In this work, heterocyclic organic derivatives namely benzimidazole, 2-methylbenzimidazole and 2-mercaptobenzimidazole were used as corrosion inhibitor for carbon steel. Polarization and electrochemical impedance spectroscopy (EIS) measurment were performed in the presence of benzimidazole, 2-methylbenzimidazole and 2-mercaptobenzimidazole at concentrations of 50, 100, 150, 200 and 250 ppm. The results indicat that the inhibitors are adsorptive- type inhibitors in both media. The mechanism of inhibition is assumed by adsorption at metal-solution interface and blocking the anodic and cathodic active sites. The inhibition efficiency increased with inhibitor concentration. The higher values of equilibrium constant and standard free energy of adsorption were obtained at the presence of 2-mercaptobenzimidazole indicating its higher adsorptivity and higher inhibiting effect comparing benzimidazole and 2-methylbenzimidazole. The adsorption of the inhibitors obyes Langmuir adsorption isotherm. The values of standard free energy indicat that the physical adsorption is more probable. The inhibition efficiency of benzimidazole and its derivatives in hydrochloric acid was decreased with temperature at the rang of 25 - 55 ?C. The activation energy of corrosion in presence of inhibitors was more than that at the absence. Indeed, the enthalpy value of adsorption indicates that the physical adsorption is occured. The synergistic effect of bromide ion are observed in the presence of benzimidazole and 2-methylbenzimidazole in sulfuric acid while this effect was negligible in the presence of 2- mercaptobenzimidazole. The synergistic effect of bromide ion indicates that the adsorption of benzimidazole and 2-methylbenzimidazole is physical while the chemical adsorption is more expected to occur in the presence of 2- mercaptobenzimidazole. The inhibition effects of benzimidazole and 2-methylbenzimidazole in 1M HCl is higher than that in 0.5M H2SO4 solution while the effect of 2-mercaptobenzimidazole in both media was almost unchanged. This result indicates that the inhibition effects of 2- Key words Benzimidazole, Corrosion inhibitors, Acid solution, Carbon steel, Adsorption, EIS