The effect of isomers of 2,5-bis(n-pyridyl)-1,3,4-thiadiazole (n-PTH) on the corrosion of mild steel in perchloric acid solution (1 M HClO4) has been investigated using weight loss, electrochemical impedance spectroscopy and potentiodynamic polarisation. Inhibition efficiency is dependent upon the thiadiazole structure, with 3-PTH serving as a better inhibitor than 2-PTH. 2-PTH stimulates corrosion at low concentrations. Polarisation curves indicate that 3-PTH acts as an anodic-type inhibitor whereas 2-PTH is of the cathodic-type. X-ray photoelectron spectroscopy were also carried out to establish the mechanism of corrosion inhibition of 3-PTH and stimulation corrosion of 2-PTH of mild steel in acid solution. Adsorption of 3-PTH on the mild steel surface in 1 M HClO4 follows the Langmuir adsorption isotherm model. (C) 2002 Elsevier Science B.V. All rights reserved.