Well-dispersed crystalline tin dioxide (SnO2) nanoparticles were synthesized by a novel and simple water-m-oil (w/o) microemulsion-assisted hydrothermal process, using low-cost tin chloride as the starting material. The typical quaternary microemulsions of cetyltrimethylammonium bromide (CTAB)/n-pentanol/n-hexane/water were used as space-confined microreactors for the nucleation, growth, and crystallization of SnO2 nanoparticles under hydrothermal conditions. The techniques of XRD, TEM, HRTEM, SAED, EDS, FTIR, XPS, and N-2 adsorption measurement were used to characterize the compositions and structures of obtained samples. The results show that the SnO2 nanoparticles have high specific areas (107-169 m(2) g(-1)), small particle sizes (ca. 3.0 nm), high crystallinity, and narrow size distributions. The well-dispersed, uniform, and well-crystallized powders with microporous texture are favorable for gas-sensing applications. The as-developed microemulsion-assisted in situ crystallizing process can be extended to prepare other oxide, metal, and sulfide nanoparticles. (C) 2004 Elsevier Inc. All rights reserved.