ZnO nanowires were grown by metallorganic chemical vapor deposition from zinc acetylacetonate hydrate and oxygen without using any seed particles or templates. Nanowires grew epitaxially on sapphire substrates to form dense arrays but with random orientation on F:SnO2 and glass. Nanowire morphology was studied as a function of deposition conditions, including substrate temperature, zinc partial pressure, and growth duration. Water evolves from the initially hydrated precursor during the first 30 min of growth, resulting in deposition of a thin polycrystalline film. The grains of this film then act as nucleation sites for nanowire growth from the now-anhydrous precursor. After several hours of growth, the precursor decomposes to ZnO, resulting in nucleation of smaller secondary nanowires on the sides of the first nanowires. These branched structures with high surface area may have potential applications in dye-sensitized solar cells and sensors. Transmission electron microscopy shows that the nanowires are crystalline, and photoluminescence is nearly free of defect emissions.