ZnO was synthesized by the hydrothermal method using proper aqueous solutions of ZnCl2 and NaOH, as the main raw materials, corresponding to the molar ratio of Zn2+ : OH- = 1 : 20 (solution ‘A’), and a proper proportion of water as solvent, ethanol (EtOH) as non solvent and polyethylene glycol (PEG) as nonionic surfactant (solution ‘B’). The reaction takes place in an autoclave at 200 °C for a defined period of time (1-20 h). The solid ZnO products received after centrifugation, washing and drying were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The percentage of unreacted Zn present after the reaction in the liquid phase (incumbent solution) was determined by Atomic Absorption Spectroscopy. The parameters of processing (i.e., temperature, pressure, residence time) and the raw materials parameters (i.e. proportion of , amount of PEG) influence the morphology and the dimensions of the product. Increasing the residence time from 1 h → 3 h → 20 h, the amorphous regions illustrated in SEM images are decreased and the number and dimensions of the obtained single or branched rods of the final product are increased. Avoiding the use of additional water, i.e. the denominator in the aforementioned ratio, mL, water, is equal to zero, but simultaneously using ethanol, the ZnO material is mainly amorphous. Increasing the amount of the additional water without ethanol, no amorphous material is observed and single rods or flowerlike/starlike structures having ends of regular hexagonal pyramid structure, are formed. Increasing the proportion of PEG, many flowerlike or starlike branched structures having ends of regular hexagonal prismatic structure, are formed. The ZnO materials having much of amorphous regions do not show XRD peaks. On the other hand, the crystalline ZnO materials show many XRD peaks, which are indexed and they correspond to the wurtzite-structured (hexagonal) ZnO. Furthermore, the lattice constants and c are determined.