Modelling of Liquid-Vapour Equilibria in the H2O-CO2-NaCl and H2O-H2S-NaCl Systems to 270 degrees C - An unsymmetric thermodynamic model for the liquid-vapour equilibria in the H2O-CO2-NaCl and H2O-H2S-NaCl systems for temperatures below the critical point of water (250 degrees C for H2S, 270 degrees C for CO2) is presented. The vapour phase is described by a cubic Equation of state. The water and gas components in the liquid aqueous phase are respectively described by Raoult's lam, and Henry's law combined with a Redlich-Kister's model of regular solutions for the activity coefficients of these two components. After an analysis of the experimental data base, more than 80% for CO2 and 92% for H2S of predicted pressures of aqueous isopleths deviate less than 5%, which is comparable to experimental uncertainty. Although the model is not fitted on the composition of the vapour phase, the predicted values are correct above 100 degrees C. The salt effect is modelled by a combination of the model of Pitzer for the water activity and an extension of Setchenow's law. The hypotheses behind this model makes it applicable at pressures below 300 har.