The role of ring substituents on the effectiveness of benzotriazole (BTA) in inhibiting the corrosion of iron in acid medium is discussed. At 0.1 mM concentration all the substituted benzotriazoles examined are considerably more effective than BTA. Minima in the corrosion rate of iron were observed at 3.0 and 4.7 mM for the benzotriazole and tolyltriazole respectively and at saturated concentrations for the other substituted benzotriazoles. At concentrations of maximum corrosion inhibition the degree of inhibition was found to increase in the order: 5-NO₂ É46 5-C1 É46 mixture of 4and 5-COOH É46 5-COOH É46 BTA É46 5-CH₃ É46 5-NH2 while, on the basis of the quantity of organic compound required for inhibition, carboxybenzotriazole is judged the most effective derivative. -- AATA