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The interaction of Zinc(II) tetraphenyl porphyrin (ZnTPP) and pyrene with colloidal TiO2was studied by absorption and fluorescence spectroscopy. Upon excitation of its absorption band, the fluorescence emission of ZnTPP and pyrene was quenched by colloidal TiO2. The bimolecular quenching rate constant (kq) is 4.09 × 1011M−1s−1for ZnTPP and 2.09 × 109M−1s−1for pyrene. ZnTPP and pyrene adsorbed on colloidal TiO2can participate in the sensitization process by injecting electrons from their excited states into the conduction band of TiO2. The mechanism is discussed on the basis of free energy change (ΔGet) obtained from Rehm-Weller equation considering quenching rate constants and conduction band potential of colloidal TiO2.