The triazole-acetate derivative TMCA was synthesized and its inhibiting action on the corrosion of mild steel in sulphuric acid was investigated by means of weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electronic microscope (SEM). The potentiodynamic polarization measurements show that the TMCA acts essentially as a mixed-type inhibitor, and EIS confirmed that changes in the impedance parameters were due to surface adsorption. The inhibition efficiencies (IE%) obtained from weight loss measurement and electrochemical tests were in good agreement. Substitutional inhibition mechanism of mild steel was established according to the fitted results at different temperatures. The number of water molecules (X) replaced by a molecule of organic adsorbate was determined from the different substitutional adsorption isotherms applied to the data obtained from weight loss experiments.