In a recent study, we proposed a supercritical water crystallization method for production of metal oxide particles. Around the critical point, the morphology of boehmite (AlOOH) particles varied greatly with the reaction temperature, pressure and concentration of aqueous aluminum nitrate solution. In this study, the relationship between the morphologies of particles obtained and the chemical species in solution is discussed. For estimation of chemical species concentrations, evaluation of equilibrium constants of the hydrothermal reactions around the critical point is required. For this, a model based on the Gibbs energy change by temperature, solvent effects and ion-ion interactions is employed. The solvent effect was calculated by the Born equation. The effect of ion-ion interaction was calculated by the extended Debye-Huckel equation. Using this model, the distribution of chemical species for the AlOOH system (Al3+, Al(OH)(2+), Al(OH)(3), Al(OH)(3), Al(OH)(4)(-), NO3-) in subcritical (350 degrees C, 30 MPa) and supercritical water (400 degrees C, 30 MPa) was estimated. The particle morphology seems to be determined by selective adsorption of positive charged species, Al(OH)(2+), on the negatively charged faces of AlOOH crystal.