Abstract A model is developed for the prediction of H 2S solubility in NaCl-bearing aqueous solutions up to about 300 bar, 320°C and 6 m NaCl. Although many HzS solubility measurements have been published, most of them are limited to low pressures (just above water saturation pressure). In order to predict solubilities to higher pressures, we have developed an equation of state (EOS) for the H 2SH 2O binary, which allows us to calculate chemical potentials of both H 2S and H 2O in liquid or in vapor phase. Below 350°C NaCl has negligible effect on the vapor phase but has substantial effect on the activities of H 2S in the liquid phase. This salt effect is accounted for by specific ion interactions. Comparison of this model with all the available data indicates that predictions are within experimental uncertainty, even for pressures well above that covered by the data used for parameterization.