Using the Taylor dispersion method, diffusion coefficients have been measured for 5,10,15,20-tetraphenyl-21H,23H-porpine (TPP), 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 2,3,5,6-tetramethyl-1,4-benzoquinone (TMBQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (TCBQ) and 2,6-di-tert-butyl-1,4-benzoquinone (DTBBQ) in hexane, decane, tetradecane, hexadecane, acetonitrile, benzene and toluene at 298.2 K. When the logarithms of the diffusion coefficients are plotted against the logarithms of the solvent viscosity, good linear relationships are obtained for TPP in all solvents and for the quinones in alkanes. For quinones, particularly BQ, NQ, and TCBQ, diffusion coefficients are found to be smaller in acetonitrile, benzene, and toluene than in alkanes when compared at the same solvent viscosity. Diffusion coefficients were also measured for zinc 5,10,15,20-tetraphenylporphine (ZnTPP) in benzene, toluene and acetonitrile, and for 9,10-diphenylanthracene (DPA) and maleic anhydride (MA) in acetonitrile. Sums of the diffusion coefficients for fluorophores and quenchers are found to be larger than those obtained from the transient effect of bimolecular fluorescence quenching reactions. Some possibilities for the differences are discussed.