Oxyfuel fired combustion has the potential to increase the fireside corrosion rates comparing to air firing combustion as a result of the increasing amounts of aggressive combustion products due to recycling of flue gas. The changes in the combustion gas chemistry will also affect the chemistry and formation of deposits, with potentially increasing corrosion and internal attack of the boiler components that are in contact with the combustion and flue gas environment. As the currently available information on the corrosion rates under oxyfuel conditions is still limited, and partially also contradicting, corrosion testing of existing boiler materials under high carbon dioxide combustion environments with the relevant oxygen, water vapour and impurity concentrations and deposits is very much needed. The work describes the laboratory testing of selected alloys (EN 1.4922, UNS S34710, UNS S31042, UNS S31035, A263 and A617) under simulated oxy- and air-firing combustion conditions with and without calcium carbonate–calcium sulphate deposit at 600 and 650 °C. The results showed that the corrosion resistance increased when the chromium content increased but without added impurities like sulphur and chlorides, the simulated oxyfuel conditions did not result in more severe corrosion than under air-fired environment. No carburisation of the metal substrate was observed after exposure to simulated oxyfuel gas atmospheres without deposit, although some carbon enrichment was detected near the oxide–metal interface. With extended exposure time, the oxide scale properties may change to enable metal carburisation. The exposure with deposit at 650 °C resulted in corrosion of all tested alloys and clear carburisation of steels EN 1.4922 and UNS S34710. Corrosion and microstructural changes, like carburisation, may reduce mechanical performance such as creep and/or fatigue strength and ductility, and thus the expected component life.