This paper describes an experimental study of the detailed influences of three major environmental parameters, temperature, carbon dioxide and oxygen, on the vapourside corrosion of Cu/Ni. The Cu/Ni materials in the investigation contained between 10 and 64% nickel but with emphasis on Cu/10Ni-base and Cu/30Ni-base alloys which are widely employed for heat-transfer tubes. Electrochemical techniques were used to investigate the corrosion behaviour of these alloys in distilled water at temperatures of 20-90 degrees C with, and without, the injection of carbon dioxide and oxygen. In the less-aggressive conditions of low temperature and pH=6, there was a general trend of improved corrosion resistance with increasing nickel content of the alloy but the distinctions between the Cu/10Ni and Cu/30Ni alloys were observed to decrease in more aggressive conditions (such as higher temperatures and in the presence of dissolved carbon dioxide). The work confirmed that the combined presence of dissolved carbon dioxide and oxygen represents a relatively aggressive environment for copper-nickel alloys but it was also demonstrated that carbon dioxide in conjunction with extremely low oxygen concentrations can exert a significant deleterious influence on the corrosion behaviour of these alloys.