Ultrafine ZnO particles were synthesized by ammonia-catalyzed hydrolysis of zinc di-n-butoxide (ZDB) in polyoxyethylene (6) nonylphenyl ether (NP-6)/cyclohexane solutions in both the reversed and swollen micelle regions of the solubilization diagram of aqueons NH3. In all micelle regions, rodlike ZnO particles were produced which were determined to be hexagonal single crystals having a wurtzite structure. The effects of changing R-w (the molar ratio of water to surfactant = [H2O]/[NP-6]), R-a (the molar ratio of ammonia to surfactant = [NH3]/[NP-6]), and ZDB on the length ([P-L]) and width ([P-W]) of the ZnO particles were examined. The values of both [P-L] and [P-W] increased slowly from about R-w = 4. This increase reflected a change in the system, from reversed micelles with water bound to the polar groups of the surfactants to swollen micelles with semibound water interacting with hydrated polar groups. At a constant R-w (R-w = 6.0), as R-a was increased, both [P-L] and [P-W] slowly decreased. The [P-L] and [P-W] profiles of the ZnO particles at various R-w or R-a values were similar to the micellar droplet size profile at various R-w or R-a values, respectively. These facts indicate that the particle size of ZnO is controlled by both the solubilized states of water and the size of the micellar droplets. However, the [P-L] and [P-W] Of the synthesized particles were independent of the ZDB. The [P-L] and [P-W] values also correlated to the average crystallite sizes of the (002) and (110) faces of the particles as determined by X-ray diffraction measurements. The long side of the rodlike particle was also found to be parallel to the c-axis of the hexagonal system.