Abstract: The thermodynamics of aqueous NaOH/KOH solution at high temperatures and pressures in a hydrothermal vessel was studied.The Pitzer model for apparent molar volume was used to investigate the relationship of filling degree and pressure.The Pitzer model for activity coefficient was modified by considering the ion association effect and the new model parameters were given.The results showed that the pressure of 10 mol·kg^-1 NaOH solution was 11.16 MPa while that of pure water was 136.7 MPa at 250℃ with the same filling degree of 90%, indicating that the pressures of alkali solutions were much lower than that of pure water under the same temperature and initial filling degree and a higher reaction temperature and a higher filling degree in the actual hydrothermal vessel was possible.In addition, the association degree of 5 mol·kg^-1 aqueous NaOH/KOH was over 30% at 100℃, and it increased remarkably with temperature and initial molality, indicating that the real concentration of OH- was much lower than the apparent molality.This work would facilitate the study of the reaction occurring in the hydrothermal vessel.