The results presented here are a part of the study aiming at control and optimisation of in-service performance of boiler materials and development of simulation tools for high temperature corrosion and oxidation of steels under service conditions (EU FP5 OPTICORR Project). The high temperature oxidation behaviour of three ferritic steels was Studied at temperatures of 500 and 600degreesC under different synthetic atmospheres simulating combustion environments. Multi-sample exposure tests in combination with SEM/EDS and XRD techniques pointed out a significant influence of the temperature and gas composition on the morphology and growth rate of the scale The results showed that oxidation kinetics and mechanism are strongly dependent on gas species such as H2O, HCl and SO2. The presence of HCl in moist air at 500degreesC and 600degreesC accelerated the oxidation process of the steels studied. In contrast, the presence Of SO2 suppressed this process. The SEWEDS studies suggested that in HCl containing atmospheres the corrosion mechanism is active oxidation. The reduction in oxidation rate in SO2 containing atmospheres could be due to the presence of sulphates at the metal/scale interface which probably influences ion transport through the oxide scale.