The sonochemical reaction between Zn(NO3)2 and hexamethylenetetramine (HMT) resulted in different products depending on amounts of Zn(II) cations in the aqueous solution. Crystalline phase pure ZnO nanoparticles were obtained by using 0.05 M Zn(NO3)2-HMT. This single phase was formed by low power sonication for 20 min in an ultrasonic bath at ambient conditions and its morphology did not change for the next 60 min. Higher concentrations of precursors (0.1 and 0.5 M) led to a mixture of zinc compounds including plate-like Zn5(OH)8(NO3)2(H2O). In contrast, a mixed phase ZnO/Zn5(OH)6(CO3)2 and a single phase Zn5(OH)6(CO3)2 were formed when the precursor concentrations were decreased to 0.01 and 0.005 M, respectively. The resulting infrared absorption and ultraviolet/visible emission were dependent on the phases and defects in these zinc compounds.