The oxidation of Ni-15% CrMo alloys has been studied at 900°C in flowing and static oxygen atmospheres. In flowing atmospheres, molybdenum has no effect: all the alloys oxidize in a protective manner. However, in static atmospheres the oxidation rate of alloys with > 3% Mo eventually accelerates, and catastrophic destruction of the alloy takes place. Under these circumstances a molybdenum-rich oxide layer is detected adjacent to the alloy. When specimens are coated with Na 2SO 4 prior to oxidation, alloys containing > 3% Mo again suffer catastrophic degradation, in either flowing or static atmospheres, and again a molybdenum-rich oxide layer is observed. This suggests that the principal role of the salt coating is to prevent the escape of MoO 3 to the atmosphere. The morphology of the attack in the rapid propagation region is very similar to that obtained in pre-sulphidation/oxidation experiments in the absence of salt and that particular aspect of the reaction is not greatly affected by molybdenum; the aluminium content is more important in determining the nature of the propagation. Attack similar to that exhibited by molybdenum-containing alloys can be obtained with Ni-15%Cr binary alloys in the presence of MoO 3 vapour in the atmosphere, and this might suggest that the MoO 3 reacted with the Na 2SO 4 to produce an acid (SO 3-rich) salt, leading to acidic fluxing. However, very similar types of attack were obtained when Na 2MoO 4 was added to the Na 2SO 4, and this should not have affected the acidity of the salt at all. These experiments suggest that acidic fluxing may not be important in the hot corrosion of alloys of this type (molybdenum-containing) and that when catastrophic corrosion is observed, its initiation is probably due to the formation of a molybdenum-rich oxide layer, molten during the reaction. There appears to be a threshold molybdenum content below which attack does not occur, and this seems insensitive to an increase in the chromium content from 15 to 25%.