Design and calculation of humid air turbine cycle need saturated humidity and e nthalpy of humid air at 423?K,5?MPa. However, the models proposed by p revious workers are available only up to 373?K,5?MPa. In this paper,a method based on Helgeson Model is proposed.The saturated composition o f N 2-H 2O system at 336.15-636.35?K,1.74-25.6?MP a and O 2-H 2O system at 373-533?K,0.99-5.77?MPa are predicte d. The deviations from the experimental data are less than 5%. The p redictions for humid air at 273-372?℃,0.1?MPa are comparable wi th the results using the Hyland and Wexler model. The saturated compositio n and enthalpy of humid air at 473-573?K,5-15?MPa,are also
