In this paper, corrosion inhibition by chemisorbed CO, at high pressure and temperature, on a high-Ni ferrous alloy (Incoloy 825) and two high-Fe alloys (13Cr-L80 (Uniloy-420) and N80 steels) in very aggressive conditions (15% (w/w) HCl solution) is described. CO was either directly dosed into the electrolyte, or produced by dehydration of formic acid. It is shown that CO is a very good corrosion inhibitor, the inhibiting effect being even higher at high pressure and temperature than at normal pressure and room temperature. The effect of combining CO with a common acid corrosion inhibitor, trans-cinnamaldehyde (TCA), at high pressure and temperature, was also studied. Under these conditions, the polymerization of TCA may be favored, leading to a thin film on the metal surface that appears to serve as a barrier to corrosion. It was found that, when CO is used in combination with TCA, the inhibiting effect of the latter is considerably intensified. (C) 2013 Elsevier Ltd. All rights reserved.