The evolution of Zn(OH)(4)(2-) solution during hydrothermal processing was studied using in situ calorimetry and the solid products were characterized by X-ray diffraction, scanning electron microscopy. It was shown that the formation of flower-like ZnO nanostructures exhibited first endothermic and then exothermic processes, of which the associated heat effect was measured as being -50.45 +/- 0.01 J mol(-1). Flower-like ZnO nanostructures and ZnO microrod may be selectively formed simply based on the hydrothermal reaction temperatures. The mechanism for the flower-like ZnO nanostructures and ZnO microrod has been suggested. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.