The charge carrier kinetics of hydrothermally treated TiO₂ nanoparticles, consisting of interconnected anatase and rutile crystallographic forms, was investigated using a heterodyne transient grating technique to obtain direct evidence of the enhancement of charge separation efficiency. We found that surface recombination arising from trapped electrons was retarded, compared with that of P25 TiO₂ nanoparticles, with the aid of an increase of particle interfaces. This means that the charge separation efficiency of hydrothermally treated TiO₂ nanoparticles is higher than that of P25 TiO₂ nanoparticles, to which the enhanced photocatalytic performance of the hydrothermally treated TiO₂ nanoparticles could be attributed.