Inhibition of mild steel corrosion in deaerated 0.5 M sulfuric acid solutions containing various concentrations of indole-5-carboxylic acid is studied in the temperature range from 25 to 55 degrees C using weight-loss, potentiodynamic and spectrophotometric tests. The adsorptive behaviour of inhibitor is also investigated using electrochemical impedance spectroscopy measurements. The indole-5-carboxylic acid is found to shift the corrosion potentials towards more noble values. This shift indicates that the addition of inhibitor mainly affects the anodic process, raising the anodic overpotential more than that of the cathodic, i.e. the indole-5-carboxylic acid behaves as mixed-type inhibitor. Because the cathodic Tafel slopes for hydrogen reduction (b(c)) are affected, the inhibitor probably affects the hydrogen reduction mechanism. The activation energy values (E-a) indicate that the indole-5-carboxylic acid increases the activation energy of the corrosion reaction. The adsorption behaviour of indole-5-carboxylic acid follows Langmuir's isotherm. Both the low values of -Delta G degrees(ads) and its decrease with temperature suggest physical adsorption. Double layer capacitance-potential curves indicate considerable adsorption of the inhibitor in the potential range (-400 to -800 mV/(SCE)). (c) 2005 Elsevier B.V. All rights reserved.