Nanocrystalline anatase titanium dioxide films were formed by a hydrothermal treatment at temperatures below 100 degrees C. The microstructural evolution of the crystallites was monitored by high-resolution transmission electron microscopy (HRTEM). HRTEM analysis of the as-grown film revealed the presence of nanocrystalline brookite TiO2 within an amorphous matrix. After 90 min of the hydrothermal treatment, the TiO2 nanoparticles remained of brookite form and a small increase in grain size was observed. Intergranular amorphous TiO2 between the nanoparticles was observed in both as-grown and 90-min-treated films. However, after further 90 min of the treatment, TiO2 nanoparticles were transformed into anatase phase with a rapid growth in grain size.