The photophysical properties of the J-aggregate of 3,3'-di(3-sulfopropyl)-4,5,4',5'-dibenzo-9-phenyl-thiacarbocyanine triethylammonium salt in the absence and presence of TiO2 colloids have been studied using UV-visible absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and ESR spectroscopy. The fluorescence emission of the J-aggregate decreases with increasing concentration of TiO2 colloids. The average fluorescence lifetime of the J-aggregate in the presence of TiO2 colloids is shorter than that in the absence of TiO2 colloids. A strong photoinduced ESR signal has been observed during illumination by light with lambda = 633 nm in the presence of TiO2 and the ESR signal can be attributed to the J-aggregate radical cation. From the above results, it is concluded that photoinduced electron transfer from the excited singlet state of the J-aggregate to the conduction band of TiO2 takes place and the electron transfer rate is about 1.5 x 10(8) s(-1).