The electrical conductivities of aqueous solutions of NaCl have been measured at 651 and 670 K at 28 MPa for molalities up to 1.0 mol kg -1 . These conductivities as well as the results of Hwang et al. (High Temp. High Pres. 1970, 2, 651-669), Ritzert and Franck (Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 798-808), andMangold and Franck (Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 21-27) for aqueous KCl have been fit to the conductance equation of Turq et al. (J. Phys.Chem. 1995, 99, 822-827) with a consensus mixing rule and either mean spherical approximation or Debye-Hückel activity coefficients. Except at one state point (NaCl at 670 K and 28 MPa), where the interactions are the strongest ( * ) 17.8), the simplest model that fits the experimental results reasonably well (( 2-3%) at molalities up to 4.5 mol kg -1 is one with only the limiting equivalent conductance and a pair association constant adjusted. Activity coefficients calculated with either the MSA and ionic diameters or with the Debye-Hückel equation of Oelkers and Helgeson (Geochim. Cosmochim. Acta 1990, 54, 727-738) (with no salting out) can be used with similar accuracy. At high concentrations this model predicts strong redissociation of the ion pairs that form at low concentrations. Oelkers and Helgeson (Geochim. Cosmochim. Acta 1993a, 57, 2673-2697) proposed a model with substantial multi-ion association (triplets and quartets, etc.). This model has two more adjustable parameters and does not fit the data without the physically unrealistic salting out coefficient used by Oelkers and Helgeson (Geochim. Cosmochim. Acta 1991, 55, 1235-1251), so this model is not recommended. For the point with the highest *, more complex models are needed at concentrations above 0.05 mol kg -1 . Good fits to the data were found for multi-ion association models (five adjustable parameters) and reasonable fits for more complex activity models (four adjustable parameters) with only pair association, so that the models are about equally accurate at equal complexity. The cluster model of Laria et al. (J. Chem. Soc., Faraday Trans. 1990, 86, 1051-1056 for the restricted primitive model is consistent with the qualitative predictions of our preferred model with only pair association.