The factor f(w)(T,p) by which air increases the saturation vapor pressure of water has been measured accurately for temperatures of 20, 50, and 75 degrees C and pressures to 14 MPa. An account is given of the measurements and of the analysis of the results to yield the interaction second virial coefficient B-aw(T) and the first experimental values of the interaction third virial coefficient C-aaw(T) of the vapor-gas mixture. When a Lennard-Jones potential is fitted, the three values of B-aw are found to be highly mutually consistent, allowing that coefficient and its derivatives to be calculated for a wide range of temperatures. The three values of C-aaw define the magnitude of this coefficient well for temperatures from about 0 to 100 degrees C, but for want of a reliable theoretical interpretation yield little information concerning its derivatives. The results for B-aw(T) are compared with published experimental values, and those for both B-aw(T) and C-aaw(T) with published theoretical estimates. The work allows the vapor-pressure enhancement factor f(w)(T,p) and the vapor-concentration enhancement factor g(w)(T,p) to be calculated for a wide range of conditions, and substantially improves the accuracy with which the thermodynamic properties of the water-air system can be calculated.