Spherical ZnO microcrystals obtained by spray pyrolysis and thermal decomposition methods as well as rod-like ZnO particles (prismatic and needle shaped) prepared from precipitation in aqueous solutions, have been characterized by electron microscopy, X-ray diffraction and infrared spectroscopy. Very different sizes of ZnO particles were obtained from spray pyrolysis. However, only the larger particles (0.7 μm) were found to be slightly deformed by infrared spectroscopy. From thermal decomposition of zinc acetate, fine particles of average size 0.05 μm, rather spherical and agglomeration free were obtained. The role of initial size and morphology in the thermal evolution is fundamental: very fine spherical particles (0.01–0.02 μm), can be sintered to give particles of 0.1–0.3 μm at 875 °C with unchanged morphology. When the temperature induces a change in spherical shape, the first microstructural changes appear to take place through the crystallographic c-axis. However, for rod-like particles, changes begin from the a, b axes, being faster for needle-shaped microcrystals.