The inhibition efficiency of 5,5 diphenyl 2-thiohydantoin (DPTH) against mild steel (MS) corrosion has been investigated in 0.1 M HCl solution by using electrochemical techniques and quantum chemical calculations. The electrochemical measurements indicate that the inhibition efficiencies increase with the increasing concentration of DPTH. According to polarization measurements DPTH retards both anodic and cathodic reactions. Thermodynamic parameters and activation energy (Ea) for the MS corrosion have been determined from electrochemical results and discussed. Furthermore, quantum chemical parameters of DPTH have been calculated to explain the corrosion inhibition mechanism. (c) 2016 Elsevier B.V. All rights reserved.