TiO2 nanoparticles were prepared with strict control of the size, molecular mass (PDI = 1.02), and mass density by hydrolyzing a precursor, Ti(acac)(2)(biphen), in the presence of a well-defined hollow Pd12L24 spherical endotemplate. The formation of highly monodisperse TiO2 nanoparticles was confirmed by H-1 NMR spectroscopy, MALDI-TOF mass spectrometry, transmission electron microscopy, and atomic force microscopy. The template was easily removed by treating the sphere with a metal scavenger followed by calcination to give crystalline TiO2 nanoparticles (2.0 +/- 0.2 nm in diameter), which showed photocatalytic activity.