A simple estimation model is proposed for describing dissociation constants of alkali-metal salts and dissolution constants of metal oxides in subcritical and supercritical water. The required parameters of the model are the Gibbs free energy, enthalpy, heat capacity, and conventional Born coefficient of the dissolved species at a given reference state. The g function needed for calculation of the temperature- and pressure-dependent conventional Born coefficient was reevaluated by regression with new experimental data of NaCl and KCl dissociation constants. The model shows a good agreement with the recently reported metal salt dissociation constants at near-critical and supercritical regions compared with the revised Helgeson-Kirkham-Flowers model.