Zinc-tetraphenylporphyine (ZnTPP) in aerated solution has been examined at room temperature.The fluorescence quenching follows the Stern-Volmer equation as a function of fluorescence intensity or fluorescence lifetime

but a smaller quenching constant K

q

is derived from the fluorescence lifetime as compared with that derived from the fluorescence intensity. This difference seems to be related to the self-absorption in intensity measurement. The experimental results reveal that the quenching constant is related with the solvent viscosity rather than the solvent polarity. This fact appeares to be an indication that this quenching is a dynamic process in nature controlled by diffusion. The analysis of the reaction kinetics shows that this is a strong quenching and the charge transfer quenching rate is larger or approximate to that of the decompsition of the formed complex. The comparison of the experiment results in this paper with the results reported previously by us and others leads us to a conclusion that the solvent effect of charge transfer quenching of excited porphyrin derivatives by quinone compounds is different according to the molecular conditions. The solvent effect on the intramolecular charge transfer and the intermolecular charge transfer accurred in triplet state are mainly due to the solvent polarity

but the intermolecular charge transfer quenching from the excited singlet state is controlled by diffusion and influenced by the viscosity of the solvent.