Four organic inhibitors namely 2-amino-1,3,4-thiadiazole (AT), 5-methyl-2-amino-1,3,4-thiadiazole (MAT), 5-ethyl-2-amino-1,3,4-thiadiazole (EAT) and 5-propyl-2-amino-1,3,4-thiadiazole (PAT) were synthesized and their influence on the inhibition of corrosion of mild steel (MS) in hydrochloric acid was investigated by weight loss and potentiodynamic polarization techniques. The inhibition efficiency (IE) of these compounds was found to vary with the inhibitor concentration, immersion time, acid concentration and solution temperature. All the investigated thiadiazoles exhibited good inhibition efficiency (IE) for mild steel in hydrochloric acid solution. The adsorption of all these compounds on mild steel in acid solution has been found to obey Langmuir's adsorption isotherm. The potentiodynamic polarization data showed that the inhibitors are of mixed type. Various thermodynamic parameters (E-a, Delta G(ads), Delta Q, Delta H, Delta S, t(1/2)) have also been calculated to investigate the mechanism of corrosion inhibition.