Two newly synthesized ligands based on 1,3,4-thiadiazolethiosemicarbazone have been isolated by the condensation reaction of 2,3-disubstituted-5-acetyl-1,3,4-thiadiazole derivatives with thiosemicarbazide in acidic medium in addition to their Co(II) chelates. The synthesized cobalt chelates that have been obtained by the reaction of each ligand with cobalt acetate were confrmed to have the formulae [(LM)Co(OAc)(H2O)2]H2O (LM–Co) and [(LN)Co(OAc)(H2O)2]0.5CH3OH (LN– Co); where LM and LN are 1,3,4-thiadiazolethiosemicarbazone ligands with methyl and nitro substituents, respectively. Comparison of the IR spectrum of each ligand with that of its cobalt complex implied that both ligands acted as monobasic tridentate connecting to the cobalt ion through N atoms of both azomethine group and thiadiazole ring and S atom of deprotonated SH group as well. The two complexes have been proved to have octahedral geometrical structures. The synthesized compounds were studied as corrosion inhibitors for carbon steel in molar hydrochloric acid solution using several chemical and electrochemical techniques. The investigational outcomes displayed that the inhibition efciencies of the examined compounds were found to augment as the concentrations of such compounds raised. At comparable inhibitors concentration, the inhibition efciency was a little increased following the order: LM>LM–Co>LN>LN–Co. The acquired high inhibition efciencies of the explored compounds were ascribed to the potent adsorption of the molecules on the steel surface and construction of adherent layers. Such adsorption was found to accord with Langmuir adsorption isotherm. There is a good correlation in the results obtained from the diferent measurements used