Rapid fluorescence quenching Of S-2-xanthione by 3,3-diethylpentane has been studied in three perfluorohydrocarbons of different viscosities. The donor fluorescence decay in the presence of a quencher was fitted using the Smoluchowski-Collins-Kimball (SCK) model. The molecular parameters, R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process, kappa, were determined. The values of parameter D for all systems studied differed from the sum of the macroscopic diffusion coefficients D-x measured independently. This behavior is explained by the dependence of the molecular diffusion coefficient (as determined from the SCK model) on the distance traveled by the donor and quencher molecules during the excited donor lifetime. (C) 2004 American Institute of Physics.