The aim to reduce the CO2-emissions has triggered the evaluation of new cycle concepts for power plants. For the coal-fired power plants, the oxy-fuel firing is a promising option for CO2- emission reduction. Here, the combustion takes place in a nitrogen-free atmosphere. The oxygen is separated from the air and burned in near-stoichiometric conditions with the fuel. The gas composition is significantly changed, when the combustion is changed from air-fired to oxy-fuel fired condition. For lignite, the carbon dioxide content is raised from 15 to 59vol% and the watercontent from 10 to ~32%. For the same fuel, the SO2-content in the flue gas increases by a factor of 3-4 to ~0.5%. These changed environmental boundary conditions will affect corrosion life of the materials especially on the water walls and the heat exchanger surfaces. Considering the significant changes in the combustion gas, the composition and the occurrence of the corrosive deposits has been evaluated with the thermodynamic modelling program ‘FactSage’. The chemical compositions of the deposits have been modelled for dried lignite from Germany. The results exhibit that the oxy-fuel firing will give a significant change in the atmosphere as well as in the deposit composition. Consequently, the corrosion rates of current used materials in air fired boilers need to be evaluated for the application in oxy-fuel fired boilers.