The electrical conductivities of aqueous solutions of Na2SO4, H2SO4, and their mixtures have been measured at 373-673 K at 12-28 MPa in dilute solutions for molalities up to 10(-2) mol kg(-1). These conductivities have been fit to the conductance equation of Turq et al.(1) with a consensus mixing rule and mean spherical approximation activity coefficients. Provided the concentration is not too high, all of the data can be fitted by a solution model that includes ion association to form NaSO4-, Na2SO40, HSO4-, H2SO40, and NaHSO40. The adjustable parameters of this model are the dissociation constants of the SO4- species and the H+, SO4-2, and HSO4- conductances (ion mobilities) at infinite dilution. For the 673 K and 230 kg m(-3) state point with the lowest dielectric constant, ε = 3.5, where the Coulomb interactions are the strongest, this model does not fit the experimental data above a solution molality of 0.016. Including the species H-9(SO4)(5)(-) gave satisfactory fits to the conductance data at the higher concentrations.