Conductivities of LiCl(aq), NaCl(aq), NaBr(aq), and CsBr(aq) solutions have been measured in the range of concentrations from 0.013 to 4 x 10(-8) mol.dm(-3) using a flow conductance cell at temperatures between 603 K and 674 K and pressures between 15 MPa and 28 MPa (water densities from 650 to 200 kg.m(-3)). Limiting equivalent conductances and ionization equilibrium constants calculated from the concentration dependence of the equivalent conductance are reported. Previous measurements of equilibrium constants in the low-density supercritical region, although of lesser accuracy, agree well with the present results. Even at the critical density and 2.5 K above the critical temperature there is no evidence for the critical effects which make the Debye-Huckel limiting law invalid at the critical isotherm-isobar. No critical scaling of ho is observed. Walden’s rule is not obeyed and the Stokes radius increases by 70% near the critical region as the density goes from 700 to 200 kg.m(-3), suggesting an increase in the number of water molecules carried along with the ion. The differences in mobility between different ions are rather small when compared to the differences in crystallographic ionic radii. The association constants are in the order CsBr < NaBr < NaCl approximate to LiCl.