Viscosities of nine (1.5, 3, 5, 7, 10, 15, 20, 23, and 26) mass% of aqueous Na2SO4 solutions have been measured in the liquid phase with a capillary flow technique. Measurements were made at five isobars 0.1, 10, 20, 30, and 40 MPa. The range of temperatures was from 298.15 to 573.5 K. The total uncertainty of viscosity, pressure, temperature, and concentration measurements was estimated to be less than 1.5%, 0.05%, 15 mK, and 0.015%, respectively. The reliability and accuracy of the experimental method was confirmed with measurements on pure water for four selected isobars 5, 10, 20, and 40 MPa and at temperatures between 296.7 and 573.7 K. The experimental and calculated values from IAPWS (International Association for the Properties of Water and Steam) formulation for the viscosity of pure water show excellent agreement within their experimental uncertainty (AAD = 0.41 %). The temperature, pressure, and concentration dependences of the relative viscosity (eta/eta(0)) where eta(0) is the viscosity of pure water are studied. The values of the viscosity A-, B-, and D-coefficients of the extended Jones-Dole equation for the relative viscosity (eta/eta(0)) of aqueous Na2SO4 solutions as a function of temperature are studied. The maximum of the B-coefficient near the 323 K isotherm has been found. The behavior of the concentration dependence of the relative viscosity of aqueous Na2SO4 solutions is discussed in terms of the modem theory of transport phenomena in electrolyte solutions. The derived values of the viscosity A- and B-coefficients were compared with the results predicted by Falkenhagen-Dole theory of electrolyte solutions and calculated with the ionic B-coefficient data. Different theoretical models for the viscosity of electrolyte solutions were stringently tested with new accurate measurements on aqueous Na2SO4. The quality and predictive capability of the various models was studied. The measured values of viscosity were directly compared with the data reported in the literature by other authors. (C) 2004 Elsevier B.V. All rights reserved.