Equilibrium partial pressures of water (p(H2O)) over solutions of (5.054 to 26.18) mol kg(-1) H2SO4 from (314.2 to 196.5) K have been determined using capacitance manometers in order to (1) obtain data for compositions relevant to the stratospheric aerosol and (2) extend and test present knowledge of the thermodynamic properties of aqueous H2SO4 at low temperature. Current approaches to calculating p(H2O) from existing thermodynamic data are critically examined and compared to measurements made in this study and others in the literature. The reliability (to within about +/- 10% at the lowest temperatures) of predictions of p(H2O) using liquid phase thermodynamic properties determined by Giauque and co-workers is confirmed for solutions from 4.37 mol kg(-1) to at least 25 mol kg(-1).