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Benzotriazole (BTAH) and copper ions exhibit synergistic effects on the electrochemical impedance spectroscopy and corrosion behavior of iron in sulfuric acid. The presence of submicro- and micromolar amounts of Cu2+ ions along with millimolar amounts of BTAH caused order of magnitude increases in the polarization resistance of the iron/acid interface, above those caused by either BTAH or Cu2+ ions (which is equivalent to order of magnitude decreases in the corrosion rate). This was accompanied by a significant decrease in the capacity of the electrical double layer of the interface, which also showed synergistic effects. The results are interpreted in terms of the deposition, on the iron surface, of a protective film composed of a polymeric complex formed from copper ions and adsorbed BTAH that is believed to be of the form [Cu(I)BTA] n . This film inhibits the cathodic partial (hydrogen evolution) reaction, which is the rate-determining step (rds) and hence the overall corrosion reaction. An increase in the concentration of either Cu2+ or BTAH enhances this synergistic effect. However, there is a critical concentration of copper ions (5 × 10-6 M) beyond which further increase decreases the magnitude of the synergistic effect. This is attributed to deposition of metallic copper on the iron surface at higher concentrations of Cu2+ ions, which promotes the rds and hence the overall corrosion reaction. The results are supported by measurements of polarization curves and mass loss under various conditions.