Unilamellar vesicles, formed spontaneously by mixing single-tailed cationic (cetyl trimethyl ammonium tosylate, CTAT) and anionic aqueous solutions of (sodium dodecyl benzene sulfonate, SDBS) surfactants have been used as reactors for the aqueous phase precipitation of nanometer sized particles within their inner cores. AlCl3 solution was encapsulated within these vesicles, aluminum ions were replaced with sodium ions in the extravesicular phase, and sodium hydroxide was then added to the extravesicular region. Hydroxyl ions penetrate through the vesicle walls and react with the available aluminum in the intravesicular region to form the product. The morphology and sizes of these particles were examined by transmission electron microscopy, while their phase and crystalline nature were probed by electron and x-ray diffraction. The product particles were nanometer-sized with near spherical morphology. Good control of particle size was achieved by varying the initial concentration of electrolyte. Single particle electron diffraction revealed a symmetric pair of spots, indicating that the particles were either single crystals or polycrystalline with á low number of grain boundaries or defects. Although wide area electron diffraction showed that the product was δ–Al2O3, powder x-ray diffraction revealed that these particles were, in fact, Al(OH)3. It is likely that heating of these nanoparticles by the high energy electron beam in a high vacuum environment causes a phase transformation, resulting in the difference between the electron and x-ray diffraction results. These results represent the first demonstration of precipitation within vesicles produced spontaneously by mixing appropriate ratios of inexpensive single-tailed surfactants, and may potentially make intravesicular precipitation a commercially viable route for making nanometer-sized particles.