One of the important and main reasons for decrease of the life time and premature depreciation of the heat resistant cast steels with long-term high temperature operations is due to simultaneous carburization and oxidation of these steels in gaseous atmosphere of CO2.Thus in order to study about this effect the CO2 gas heating-test was carried out in the laboratory and the heat resistant HH cast steel ( according to the ACI standard ) as well as two other heat resistant cast steels which had different chemical compositions were studied and tested .In order to examine the surface layers the Electron Microscope and EDAX analysis were used and to examine the variations in the structure as the result of carburization the Optical-Microscope was used also the EDAX analysis for identification of phases in structure was applied .Moreover the variations of the carbon concentration in the direction of the samples’ thickness was studied. The studies showed that the steels which have more chromium and nickel are more resistant in relation to this effect or phenomenon and have less absorption in comparison to the other samples.