Enthalpies of dilution measured using isothermal flow calorimetry are reported for aqueous solutions of NaOH, KOH, and HCl at 370 degrees C and 24.6 MPa. Previously collected data at 300, 325, and 350 degrees C were included with the new results when fitting the parameters for the Pitzer excess Gibbs energy ion-interaction equation. Equilibrium constants, enthalpy changes, entropy changes, and heat; capacity changes for the association of Na+ and K+ with OH- ion and of H+ with Cl- ion were estimated from the heat data. Both enthalpy and entropy changes are positive for all of the systems and increase dramatically as the temperature increases. The resulting equilibrium constants show significant, but smaller, increases with temperature.