The inhibition effect of a synthesized imidazoline inhibitor on carbon steel in a supercritical CO2-H2O system was investigated using weight loss measurement, electrochemical tests, and surface analyses. In the presence of the imidazoline inhibitor, the corrosion rate and corrosion process showed minimal change with increasing CO2 partial pressure under non- supercritical CO2 conditions, whereas the corrosion rate clearly increased and the corrosion process greatly differed when the supercritical CO2 condition was reached. The introduction of KI substantially decreased the corrosion rate in the supercritical CO2 condition. The excess surface charge of the carbon steel changed from negative to positive as the partial pressure of CO2 increased to the supercritical condition, thereby hindering the adsorption of imidazoline inhibitor, which in turn decreased the inhibition efficiency. (C) 2017 The Electrochemical Society. All rights reserved.