Four commercially available, water soluble corrosion inhibitors for C02 corrosion have been tested in the laborato~ on carbon steel specimens that were corroded for up to 18 days in the medium prior to inhibitor addition. The tests were performed at 20-50 “C, pH 5, 1 bar C02, 1-3 w% NaCl in glass cells and a glass loop. The results show that inhibitor performances were impaired with increasing precorrosion time and increasing temperature. The resulting corrosion attack was localised within deep pits. The detrimental effect is influenced both by the nature of the steel and the inhibitor composition. The inhibitor failure is related to the formation of a cementite layer at the steel surface.