The effect of isomers of 3-pyridyl-substituted 1,2,4- and 1,3,4-thiadiazoles (3-PTHD and 3-PTH) on the corrosion of mild steel in acidic media (1 M HCl and 0.5 M H2SO4) has been investigated using weight loss measurements and electrochemical techniques (impedance spectroscopy and polarisation curves). A comparison of the results showed that 3-PTHD was the best inhibitor in both media. It is found to behave better in 1 M HCl than in 0.5 M H2SO4. Polarisation curves indicate that 3-PTH and 3-PTHD are mixed type inhibitors in 1 M HCl, but in 0.5 M H2SO4, the inhibition mode of these inhibitors depends on the electrode potential and they act essentially as cathodic inhibitors. Adsorption of 3-PTH and 3-PTHD on the mild steel surface in 1 M HCl and 0.5 M H2SO4 follows the Langmuir isotherm model. Molecular modelling has been conducted to correlate the corrosion inhibition properties with the calculated quantum chemical parameters. (C) 2004 Elsevier B.V. All rights reserved.