Abstract：The interactions of the model compound (FL4) or diad (FL-AQ) composed of fluorescein and 2-methyl-anthraquinone and TiO_2 colloid have been examined by absorption, fluorescence spectroscopy and fluorescence lifetime. This diad (FL-AQ) or model compound (FL4) is adsorbed on TiO_2 colloid through the phenolic function of the fluorescein moiety. The apparent association constant (K_(app)) of FL4/TiO_2 obtained from the absorption spectra is ca. 3590 mol~(-1)·L. The K_(app) of FL-AQ/TiO_2 is ca. 3060 mol~(-1)·L. The degree of association (α) between TiO_2 and FL4 is ca. 0.86. The degree of association (α) between TiO_2 and FL-AQ is ca. 0.84. The mechanism of the photoinduced electron injection was also discussed through ESR spectra.