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The corrosion performance of three different superheater austenitic steels (TP347HFG, HR3C and Sanicro 25) has been determined in laboratory tests under simulated oxy-fuel conditions (60 CO2 - 4 O2 - bal N2/Ar vol%) with three levels of H2O (0, 10 and 30 vol%), with and without a synthetic carbonate based deposit (85 CaCO3 - 15 CaSO4 wt%) at 650ºC for 168, 500 and 1000 hours. The results obtained from oxy-fuel conditions are compared with corresponding tests simulating conventional air-combustion and co-combustion in air (with and without the 85CaO-14CaSO4-1KCl synthetic deposit). The results from the 168 h tests in a moist and a high CO2 atmosphere showed clear corrosion of TP347HFG. In addition, carburisation was observed already in the 168 h tests with the CaCO3-CaSO4 deposits. The HR3C and Sanicro 25 performed better than TP347HFG. However, growing oxide nodules could be observed already after 168 h in some cases with HR3C and Sanicro 25. A clear oxide layer was observed on all materials after 168 h exposures in air, while oxy-fuel conditions with water vapor caused significant corrosion only to the TP347HFG.