Selected thiourea derivatives have been synthesized and evaluated as corrosion inhibitors of mild steel in 20% formic acid by weight loss and potentiodynamic polarization methods All of these compounds have shown very good inhibition efficiency (IE) in the formic acid. IE of these compounds has been found to vary with the concentration of the compounds, solution temperature, immersion time and concentration of the acid solution. The adsorption of these compounds on the steel surface from the acid has been found to obey Temkin's adsorption isotherm. The values of activation energy (E-a) and free energy of adsorption (AGads) indicated physical adsorption of these compounds on the steel surface. The potentiodynamic polarization studies revealed that all the compounds are mixed type inhibitors. (C) 2002 Elsevier Science B.V. All rights reserved.