Zinc oxide (ZnO) thin films on c-sapphire substrates were synthesized by spin-coating aqueous polymeric precursors. The effects of annealing at 1000 degrees C on crystallinity, surface morphology, and optical properties of ZnO thin films, with varying thicknesses, were studied. Single-layered ZnO thin films are polycrystalline with wurtzite structure and preferentially oriented along the (002) plane. X-ray diffraction pattern also reveals the presence of spinel zinc aluminate (ZnAl2O4) peaks. ZnO films have highly faceted granular morphology. Multilayered ZnO films, annealed twice at 1000 degrees C, do not exhibit any ZnO peaks but only ZnAl2O4 peaks. Moreover, the surface morphology was smooth with ridges. These films do not exhibit the band gap or ultraviolet emission photoluminescence characteristics of ZnO. On annealing, there is an interfacial reaction between ZnO and sapphire resulting in ZnAl2O4. (C) 2013 Elsevier B.V. All rights reserved.