Selected oxadiazoles of fatty acids; namely 2-hepta decene-5-mercapto-1-oxa-3,4-diazole (HMOD); 2-undecane-5-mercapto-1-oxa-3,4-diazole (UMOD); and 2-decene-5-mercapto-1-oxa-3,4-diazole (DMOD), were synthesized. Their influence on the inhibition of corrosion of mild steel in 20% formic acid (HCOOH) was investigated by weight loss and potentiodynamic polarization techniques. The inhibition efficiency of the compounds was found to vary with concentration, immersion time and temperature. All the compounds showed good inhibition efficiency (e(IE)) in formic acid solution. Adsorption on mild steel obeyed the Langmuir adsorption isotherm. Potentiodynamic polarization revealed that all three inhibitors, HMOD, UMOD, and DMOD are mixed inhibitors. Electrochemical impedance spectroscopy was also used to investigate the inhibition mechanism.