The ultrasonic reaction of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (C6H12N4) was investigated by varying the concentration of the reactants, the irradiation time, and the type of sonicator. The morphology, composition, and phase structure of the products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and ultraviolet–visible (UV–vis) spectroscopy. Octahedral zinc oxide (ZnO) micropowders were formed at low concentrations, 0.05M, of Zn(NO3)2·6H2O and C6H12N4 in both lab-made sonicator and commercial ultrasonic bath. However, at concentrations between 0.1 and 1.0M Zn(NO3)2–C6H12N4 mainly plate-like zinc hydroxide nitrate hydrate (Zn5(OH)8(NO3)2(H2O)2) resulted with only a small fraction of ZnO, irrespective of the irradiation time employed, highlighting the sensitivity of the system to the concentration of the starting materials. Heat treatment of Zn5(OH)8(NO3)2(H2O)2 at 350°C in air affords a ZnO phase of irregular morphology. Octahedral ZnO is found to exhibit slightly lower IR absorption and similar UV absorption to that of commercial prismatic hexagonal ZnO, although an extra peak due to small quantities of Zn5(OH)8(NO3)2(H2O)2 is observed.