Thermodynamic stability of anatase nanoparticles and their transformation behaviors to rutile phase in an acidic solution was investigated in situ at two different peptization temperatures using a freezedrying method. When peptized at 30 degreesC, the initial product was anatase with a significantly distorted atomic structure, a significant amount of hydroxyl group and Ti3+ ions, and, thus, a thermodynamically unstable state. The instability of 30 degreesC-peptized anatase was responsible for a suitable transformation to rutile later via dissolution of the anatase to form a titanium hydroxylate, followed by reprecipitation into rutile. On the other hand, 80 degreesC-peptized anatase had a relatively more ordered atomic structure, a much reduced amount of hydroxyl group, negligible Ti(sup)3+ ions, and, thus, a thermodynamically more stable state. Plausible reasons why the 80 degreesC-peptized anatase does not transform to rutile were deduced.