Densities of Na2SO4(aq) and K2SO4(aq) have been measured by vibrating-tube densitometry at concentrations of 0.01 mol . kg(-1) to 1.0 and 0.5 mol . kg(-1), respectively, and at temperatures from 298.15 K to 572.7 K and pressures up to 30.7 MPa. Apparent molar volumes derived from these data were extrapolated using the Fitter ion-interaction model to yield partial molar volumes of these electrolytes at infinite dilution. The two salts show similar behaviors over the whole range of conditions studied.