The kinetics of rubrene (5,6,11,12-tetraphenylnaphthacene) fluorescence quenching by energy transfer to the radicals of N,N,N',N'-tetramethyl-para-phenylenediamine (TMPD+*), tetracyanoquinonedimethane (TCNQ-*) and galvinoxyl (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy; GAL*), have been measured in solvents of different viscosities. At room temperature the observed quenching rate constants vary between 0.12 and 7.8 x 10(10) M(-1) s(-1), always exceeding the diffusion rate constants given by the simplest version of Smoluchowski theory combined with the rate constants for dipole-dipole energy transfer. This discrepancy is explained by accounting for the non-stationary stage in the Förster kinetics, even though the experimental fluorescence decays are apparently mono-exponential.