5-aminoindole (AI) and 5-chloroindole (CI) were tested as corrosion inhibitors of mild steel in 1N deaerated sulphuric acid. Their behaviour was compared with that of indole (IN) in the temperature range of 25 to 55 degrees C. The inhibiting effect of AI and CI is more pronounced on the anodic rather than cathodic process with the exception of CI at highest concentrations. AI and CI generally did not affect the corrosion reaction mechanism. Corrosion rates decreased as inhibitor concentration increased, but as temperature increased higher corrosion rates resulted. At 25, 35, 45 and 55 degrees C both the adsorption of AI and CI followed Frumkin’s isotherm. The best inhibiting effect was obtained at the highest concentrations (0.005-0.0075 M) of CI up to 35 degrees C. This is probably due to the fact that this molecule, as IN, can form oligomers on the electrode surface. The Contact Electric Resistance (CER) technique, described below, was used the first time to study the interaction of organic inhibitors with a metal surface.