Laboratory exposures have been performed simulating a selective catalytic reduction (SCR) system with urea injection for NOx reduction in diesel exhaust after-treatment. The corrosion behaviour of three ferritic and one austenitic stainless steel was examined using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Continuous exposure to condensate did not cause any corrosion. Results show that cyclic interaction between high temperature and condensation aggravates the corrosion compared to isothermal exposure at 450 °C. All ferritic alloys exhibited more or less the same behaviour, while the austenitic steel performed better. In fact, the presence of urea decreased the corrosion compared to the environment without urea. The cyclic samples exposed with urea displayed iron sulphate on the surface. The sulphate appeared to decrease the oxide thickness. A sulphur enrichment in the form of sulphide also occurred in the inner chromium-rich oxides of all cyclically exposed samples, both with and without urea. Thus, sulphidation is presumed to be involved in the corrosion process.