The pH values of aqueous solutions of boric acid and KOH were measured with the optical indicator 2-naphthol at temperatures from 300 to 380 degrees C. The equilibrium constant K-b(-1) for the reaction B(OM)(3) + OH- = B(OH)(4)(-) was detennined from the pH measurements and correlated with a modified Born model. The titration curve for the addition of HCl to sodium berate exhibits strong acid-strong base behavior even at 350 degrees C and 24.1 MPa. At these conditions, aqueous solutions of sodium berate buffer the pH at 9.6 +/- 0.25.