The hot corrosion behaviour of various nickel and cobalt-base alloys has been studied using the coating test, in which alloy samples were coated with approx. 1 mg/cm 2 Na 2SO 4 and oxidized in slowly flowing oxygen at 1 atm. pressure. The reaction kinetics were followed thermogravimetrically, and the corrosion products were examined in detail using conventional metallographic techniques, X-ray diffraction and electron probe microanalysis. In both nickel and cobalt-base alloys, the presence of solid solution strengthening elements, such as molybdenum, tungsten or niobium, are important in promoting accelerated oxidation in the presence of a condensed layer of sodium sulphate on the surface of the alloy. Surprisingly, tantalum does not show a similar effect. The oxides of these elements react with the salt forming molybdates, tungstates or niobates, removing oxide ions and thereby increasing the acidity of the sulphate layer. Both NiO and CoO are then soluble in this acidified salt layer, CoO apparently more so than NiO, and accelerated oxidation can proceed. Alloys containing sufficient chromium to form a continuous. protective Cr 2O 3 layer are not subject to accelerated oxidation: acid fluxing of Cr 2O 3 does not occur. However, aluminium containing alloys which under normal circumstances are protected by an Al 2O 3 scale, do suffer accelerated oxidation because Al 2O 3 is fluxed from the surface by the molten sulphate layer. Other factors, apart from alloy composition, affect the hot corrosion behaviour. The presence of NaCl can initiate attack in normally resistant high chromium alloys by causing cracking of the protective Cr 2O 3 scale. Temperature is also important: at low temperatures the influence of the Na 2SO 4 layer is small because of the slow rates of reaction; at high temperature it is also small because the salt layer is evaporated off the surface. Locally reducing, or low oxygen potential atmospheres can also be important in promoting accelerated oxidation by Na 2SO 4, in that they allow sulphur penetration into the alloy and the resultant precipitation of chromium-rich sulphides causes a depletion of chromium such that a protective Cr 2O 3 scale is not developed.