Titanium dioxide nanoparticles prepared in water-in-oil microemulsion droplets by controlled hydrolysis of TiCl4-generated crystalline nanoparticles of sizes from 115 nm down to 6 nm diameter depending on the size of the aqueous core of the micellar droplets. Powder X-ray diffraction of the vacuum-dried product (without sintering) indicated the presence of an unusual type of orthorhombic crystal structure nearly similar to titanium dioxide crystals prepared at high pressure. On gradual heating up to 900 degreesC these metastable crystals are converted into relatively more stable nanorods perhaps through making and breaking of the Ti-O-Ti bonds. It has been concluded that chemical pressure generated within the constrained volume of aqueous core of the reverse micellar droplets is responsible for the unusual crystal structure of TiO2 nanoparticles. (C) 2003 Elsevier Inc. All rights reserved.