Crystallite size and crystallinity have been recognized as important parameters that influence the photocatalytic performance of pristine TiO2 nanoparticles. But how these two parameters individually affect the photo-reactivity of TiO2 remains unclear and debated due to the difficulties in preparing well-controlled TiO2 photocatalysts that vary crystallite size and crystallinity independently. Here, we have studied the effect of crystallite size and crystallinity on phenol photo-decomposition using well-defined anatase nanoparticles synthesized under supercritical water-isopropanol conditions. The photo-reactivity was found to increase with increasing crystallite size but to be independent of the crystallinity. By tracking the evolution of phenolic intermediates, we explored the reaction kinetics and demonstrated that an increase in the crystallite size of anatase nanoparticles can significantly suppress undesired hydroquinone-benzoquinone redox reactions and thus promote the full decomposition of phenol and phenolic compounds. (C) 2013 Elsevier Inc. All rights reserved.