A miniemulsion-based approach to produce crystalline ZnO nanoparticles with narrowly distributed sizes and well-defined faces and shapes is presented. The synthesis reaction in the aqueous droplets of the water-in-oil miniemulsion is induced by mass transfer of the base triethylamine (TEA) across the liquid/liquid interface. In a preliminary study, the dosage of TEA was determined to ensure the required pH range in the aqueous phase. Miniemulsions with a mean droplet size of about 200 nm were produced with a rotor-stator device. The aqueous zinc salt concentration was varied as the parameter in the experimental study. At low zinc salt concentrations, platelike nanopartides were obtained. An increase in the zinc ion concentration resulted in a decrease in the mean particle size. The shape of the ZnO crystals changed from platelike to rodlike. On the basis of the resulting particle size, it can be deduced that interdroplet exchange is an essential part of the presented miniemulsion-based process.