The corrosion of superheater tube material in straw-fired boilers was investigated in the laboratory. Metal test elements of boiler tube steel (X20CRMV121 and AISI 347FG) were covered with synthetic (KCl and/or K2SO4) and real deposits, and exposed to a synthetic flue gas (6 vol % O-2, 12 vol % CO2, 400 ppmv HCl, 60 ppmv SO2, balance N-2) in 550 degrees C electrically heated ovens. Exposure times from 1 week to 5 months were used. The corrosion of the metal test elements was, in general, quite uniform, and the corrosion products consisted mainly of oxides of iron and chromium. All test elements covered with KCl suffered from minor internal attack, and some elements had severe pits with chlorine found in the pit. A dense layer of potassium sulfate and iron oxide was found adjacent to the metal oxide layers on all the metal test elements covered with a deposit containing KCl. The layer had a characteristic structure, with iron oxide threads in a dense potassium sulfate matrix. A mechanism for chlorine corrosion is suggested. The mechanism is based on gaseous chlorine attack coupled with a fast sulfation of KCl to K2SO4 in a melt of KCl, K2SO4, and iron compounds formed adjacent to the metal. Aspects of the sulfation of potassium chloride to potassium sulfate are discussed in the paper.