Monodispersed titania nanoparticles were prepared from reacting TiOCl2 with NH4OH in water/Triton X-100/n-hexanol/cyclohexane microemulsions. The effect of calcination on the microstructures of the particles was investigated. The particles synthesized were amorphous, transformed into the anatase phase at 300degreesC, and further into the rutile phase at 850degreesC. The crystallite size of the particles was 9.7 to 35.6 nm in the temperature range between 300 and 900degreesC. Secondary particles, agglomerates of finer primary particles, were about 20 nm in size at 200degreesC and increased markedly by a factor of 10 to 20 at 900degreesC due to a significant interagglomerate densification. With increasing calcination temperature from 300 to 900degreesC, the specific surface area of the particles decreased rapidly from 317.5 to 8.4 m(2)/g, whereas the average pore radius increased considerably from 2.9 to 31.8 nm as the result of shrinkage of the agglomerates, destruction of the minute intercrystallite pores, and interagglomerate densification.