The equilibrium constant K-BHA for a reaction between an organic acid (beta-naphthol) and a base (OH- ion) has been measured for the first time in supercritical water (SCW) up to 400 degrees C and 470 bar, by using UV-vis spectroscopy. Solvatochromic shifts for the beta-naphtholate anion are used to determine the extent of hydrogen bonding with water and ion pairing with the series of cations Na+, K+, and Cs+. The ionization constant for beta-naphthol, K-2, is determined from K-BHA and previous measurements of the ionization constant for pure water, K-w. All of the results are consistent with the Born model. At constant temperature, density effects are much larger for K-a than K-BHA, since the latter reaction is iso-Coulombic. At constant water density, K-BHA is exothermic due to the stronger acidity of 2-naphthol versus water, whereas K-a is endothermic due to the energy required for ionization. However, the behavior becomes much more complex at constant pressure, due to large negative values of partial molar enthalpies, entropies, and volumes of ions at high temperatures, which are a result of the large isothermal compressibility and volume expansivity of the solvent.