Zinc oxide thin films were prepared by spray pyrolytic decomposition of zinc acetate onto a glass substrate. Auger spectroscopy showed that the film stoichiometry is close to the ZnO phase with a little excess of oxygen. X-ray diffraction spectra show that the structure belongs to the hexagonal wurtzite crystal type, with a mean crystallite size in the range 20-33 nm. Under optimized deposition conditions films are caxis oriented, having a full width at half-maximum (FWHM) value of the (002) X-ray diffraction line of 0.238. Microstructure was analysed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution electron microscopy (HREM). In regard to the crystal growth, a critical temperature was found to be around 600 K. Above this critical temperature the film is c-axis oriented and almost all grains became round shaped. Optical constants, n and k, were determined using only transmittance data and a direct band gap of 3.28 eV was deduced.