Loop testing has been carried out to identify the corrosion for 13Cr modified 13Cr and 15Cr stainless steels under 20 bar CO2 in 20000 ppm NaCl solution with varying concentrations of H2S and organic acids at a temperature of 40°C. It is found that the corrosion behaviors of the steels strongly depend on H2S and organic acids. The results indicate that the 13 Cr steel corrodes significantly at 20 bar CO2 conditions with H2S present. The modified 13 Cr and 15 Cr steels show very good corrosion resistances under CO2 with different H2S partial pressures from 0 to 20 mbar. However severe pitting and general corrosion are observed under the CO2-H2S environments with 0.01 and 0.02 M acetic/ propionic acids for both modified 13 Cr and 15 Cr steels.