A program of measurements of the density of binary gaseous mixtures of water with nitrogen, carbon dioxide and methane has recently been completed at temperatures up to 700 K and pressures up to 35 MPa using an isochoric method. The density has been determined in all cases with an accuracy of better than ± 0.2%. It has also been possible to determine the dew point parameters of the mixtures studied. The results have been used in conjunction with other available information to evaluate the performance of three thermodynamic models for the prediction of the phase behaviour and gas phase properties of such polar / non-polar mixtures. The two-fluid corresponding states model has proved to be capable of the most accurate predictions of the density over the complete range of conditions. The one-fluid corresponding states model yields significantly less accurate predictions but remains acceptable even under extreme conditions. However, the model due to Peng and Robinson works satisfactorily only at pressures below 30 MPa and at water mole fractions less than 0.5. Under these conditions, the density predictions are accurate to within 1%. On the other hand, the PengRobinson model provides dew-point temperatures in excellent agreement with experiment.