It is well known that water vapour accelerates oxidation; however different gas conditions and material compositions affect the mechanism. The paper addresses this issue from two different application areas; biomass and kraft recovery boilers. In these applications water vapour and sulphur are simultaneously affecting the corrosion mechanism, though the mechanisms are different. Low-alloyed steels were exposed to an atmosphere containing different amounts of water vapour at temperatures of 420, 550 and 600°C. Under oxidising conditions increasing water content generally accelerates oxidation. However, presence of SO2 in moist atmosphere retards oxidation at high temperatures. The phenomenon is seen at low temperatures with higher chromium contents. Stainless steel 304L was tested in an atmosphere containing hydrogen sulphide and carbon monoxide with and without water vapour at a temperature of 440°C to simulate elevated kraft recovery boiler furnace conditions. The tests showed that water vapour in the test atmosphere produces a protective spinel oxide on the metal surface. In tests without water vapour, the initial scales at metal surface were different sulphur compounds and intensive sulphidation occurred. The effect of water vapour on the sulphidation mechanism is addressed in the paper through the described tests and thermodynamic modelling.