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