This paper presents the results of a laboratory investigation relating to vapour-side corrosion problems that have been encountered in recent times in many large thermal desalination plants. The experiments comprised a study of the behaviour of two copper-nickel alloys in condensing-vapour conditions at 90-95degreesC. Some of the tests involved pure water vapour whilst others examined the effects of injection of carbon dioxide and air into the condensing vapour. The corrosion process was studied by means of weight-loss measurements together with microscopical examination of specimens. In pure water vapour, the corrosion resistance of a Cu-30%Ni-base alloy was observed to be substantially superior to a Cu-10Ni-base material. The injection of carbon dioxide increased the severity of the corrosion process and, of particular interest, represented environmental conditions in which the corrosion of the Cu-30Ni-base material appeared to be of a generally similar magnitude to that of the Cu-10Ni-base alloy. The microscopical examinations revealed interesting features of the morphology of the corrosion attack and this aspect of the study is discussed in relation to the basic mechanisms of corrosion. Attention is given to the role of composition on the behaviour of copper-nickel alloys in these environmental conditions and to the relevance of the findings to practical vapour-side corrosion problems in desalination plant.