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A theoretical study of benzimidazole (BI) and two of its derivatives namely 2-methylbenzimidazole (2-CH₃BI) and 2-mercaptobenzimidazole (2-SHBI) recently used as corrosion inhibitors for mild steel in 1M HCl was undertaken by considering Density Functional Theory (DFT) at the B3LYP/6-311G++(d,p) level. The properties most relevant to their potential action as corrosion inhibitors has been calculated in the neutral and protonated form: E_HOMO, E_LUMO, energy gap (ΔE), dipole moment (μ), electronegativity (χ), global hardness (η) and the fraction of electrons transferred from the inhibitor molecule to the metallic atom (ΔN). The theoretical results are in agreement with the experimental data.