This paper presents a systematic study on the role of particle size in pure and doped nanocrystalline TiO₂ photocatalysts, which was made possible by a versatile wet-chemical process capable of generating near-agglomeration-free TiO₂ with well-controlled particle sizes and dopant dispersion. It is shown that particle size is a crucial factor in the dynamics of the electron/hole recombination process. For TiO₂ particles with 6 or 11 nm diameter, Fe³⁺ dopants were added to inhibit the charge carrier surface recombination. The optimal Fe³⁺ dopant concentration for different particle sizes was identified, and this concentration was found to decrease with increasing particle size. To assist electron and hole separation in TiO₂ with 21 nm diameter, Nb⁵⁺ dopants were introduced in combination with minor surface Pt dispersion. These carefully engineered nanocrystalline TiO₂ catalysts showed higher reactivities than Degussa P25 TiO₂ material in photocatalytic decomposition of chloroform.