Hot corrosion studies using thin films of molten sulfate are reported. A specially machined crucible held coupons of 304 stainless steel totally surrounded by a I nim molten salt layer. Pre-oxidised and pre-carburised coupons were held at 650 degreesC in the melt under a synthetic flue gas for up to 2000 h and analysed at 200 h intervals for the concentrations of Fe, Cr and Ni entering the melt. After initial attack the concentration of these elements remained essentially constant for around 800 h, after which spallation of the protective Cr2O3 layer and rapid attack occurred over the next 400 h, by which time the protective layer had reformed and concentration levels remained constant for around 700 h when the process was repeated. The pre-carburised coupons produced a several-fold increase in iron, chromium and nickel concentrations in the melt over pre-oxidised coupons. This is, to our knowledge, the first time the stop-start corrosion mechanism has been identified experimentally, and confirms that increased corrosion can take place if superheater and reheater tubes made of 304 steels are subjected to reducing conditions during a failed start-up procedure or subsequently.