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The constant volume, constant composition heat capacity data for aqueous sodium sulfate show several interesting features:  one peak or step when solid salt first precipitates and, on further heating, a λ-shaped peak when the vapor- or liquid-phase disappears. These features are interpreted in terms of the solid−liquid−vapor phase diagram, and their consistency is shown with earlier PVTx, phase boundary, and three-phase data from the literature. New data are presented on the density of liquid solutions saturated with respect to the vapor. New liquid compositions or densities supplement the three-phase data available in the literature, and new fluid densities are reported near the critical end point. For aqueous Na2SO4, we find T cep = (648.2 ± 0.2) K, and for aqueous Na2CO3, T cep = (649.0 ± 0.2) K. The present heat capacity data, as well as recent data for aqueous sodium carbonate, have sufficient resolution to distinguish, for the first time, the temperature of the critical end points in aqueous sodium sulfate and aqueous sodium carbonate from the critical temperature of pure water, T c = (647.1 ± 0.1) K.