The photocatalytic activity of amorphous TiO2 (not hydrated TiO2 or titanium(IV) hydroxide) is clarified to be negligible in some deaerated suspensions, while anatase crystallites in similar particle size of ca. 30 nm show appreciable photocatalytic activity. The negligibly low activity of amorphous TiO2 is attributable to the facilitated recombination of e- and h+ at the traps on the surface and in the bulk of the particles. The predominant participation of the latter traps is suggested, but their qualitative and quantitative evaluation has not been achieved. Theseresultsgiveessentialandimportantinformation for the design and synthesis of highly active semiconductor photocatalyst. Time-resolved studies on the recombination kinetics in these TiO2 particles are in progress.