We have monitored the homogeneous crystallization of TiO2 phases from aqueous TiCl4 solutions at 100 degrees C using both time-resolved small-angle X-ray scattering (TR-SAXS) and time-resolved wide-angle X-ray scattering (TR-WAXS). The WAXS data revealed that metastable anatase was the first phase to crystallize, but it began transforming to rutile within the first hour of reaction. Scherrer analysis of WAXS peak widths indicated that anatase particles grew to an average size near 7 nm on a time scale of minutes, after which their size remained static, while rutile grew rapidly from <3 nm to similar to 10 nm within 1 h, followed by slower growth at a rate of similar to 0.15 nm/h. Using a novel sample cell designed to withstand temperatures up to 150 degrees C and hydrothermal vapor pressure, we collected in situ SAXS data for the same conditions as the WAXS experiments. Fourier analyses of the SAXS images revealed the rapid emergence of a broad, Gaussian-type particle size distribution ranging in diameter from 1 to similar to 20 nm with a mean near 7 nm, in excellent agreement with our WAXS analysis. At 0.5 h, a second population of <3 nm particles emerged, and these crystals grew to similar to 9 nm after 7.5 h, which we interpret as the nucleation and growth of rutile (as was also observed in the WAXS patterns). In contrast to studies that presume a solid-state transformation from anatase to rutile during growth, our results suggest that anatase and rutile co-precipitate during the early stages of crystallization, but once anatase crystals grow to a critical size of 7 nm, they dissolve and the solution species re-precipitate onto primary rutile nanocrystals. (C) 2012 Elsevier By. All rights reserved.