Natural inhibitors including plant extracts and their pure metabolites have become important as environmentally acceptable, low-cost, readily available and renewable source for a wide range of corrosion inhibitors. In the current study, the inhibitory effect of the methanol extract of Gentiana olivieri and its fractions on the corrosion of mild steel in a 0.5 M HCl solution were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The chemical compositions of the methanol extract and its fractions were identified by high-performance liquid chromatography (HPLC). EIS results showed maximum inhibition efficiency (93.7%) at an inhibitor concentration of 800 mg/L. Polarization curves indicated that the plant extract and its fractions are mixed-type inhibitors. Surface analyses (SEM/EDX) were carried out to establish the corrosion-inhibitive properties of the samples. Thermodynamic parameters (E-a, Delta H degrees, Delta S degrees, Delta G degrees) were also examined. (C) 2016 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University.