Zirconia nanoparticles were prepared in supercritical water using hydrothermal method, where particle morphology was found to depend on the reaction temperature and pressure, pH and precursor concentration. Having calculated the equilibrium constant using revised HKF (R-HKF) and Sue-Adschiri-Arai (SAA) thermodynamic models, the relationships between particle morphological characteristics and distribution of zirconium chemical species (including Zr4+,ZrOH3+,ZrO2, Zr(OH)(2)(2+), Zr(OH)(3)(+),Zr(OH)(4), Zr(OH)(5)(-), NO3-) in subcritical and supercritical water were studied. Results indicated that Zr(OH)(2)(2+) was the dominant species at lower pH, while at higher pH, Zr(OH)(5)(-)-became the dominant one. Distribution of zirconium species as well as different phases of ZrO2, suggested that Zr(OH)(2)(2+) and Zr(OH)(5)(-) - ions most likely play a significant role in the synthesis of monoclinic and tetragonal phases, respectively. (C) 2014 Elsevier B.V. All rights reserved.