This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO2) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO3)(2)) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250 degrees C for 24 h to obtain pure monoclinic-structured ZrO2 nanocrystals. The crystallization temperature of the ZrO2 phase was also greater than 200 degrees C. However, the products of reactions conducted at 200 degrees C for 24 h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250 degrees C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12 h in order to obtain single-phase monoclinic ZrO2 nanocrystals. The crystallite size of this product was approximately 20 nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface. (C) 2013 Elsevier B.V. All rights reserved.