The quenching of the fluorescence decay of electronically excited 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonate (TPPS4-*) in the presence of methylviologen cations (MV2+) was measured at various ionic strengths in methanol. Analysis of the fluorescence decay curves revealed strong evidence for the presence of a second fluorescent species over the entire range of ionic strength used in this work, which is attributed to solvent-separated ion pairs (TPPS4--S-MV2+). Transient effects of the fluorescence decays were analyzed, and values for the effective reaction distance, R-NH, and the diffusion coefficients, D, were obtained. Diffusion coefficients were independently measured for TPPS4- and MV2+ using the Taylor dispersion method. The values for D obtained by the analysis of the transient effect were found to be smaller than those for the sum of the diffusion coefficients of TPPS4- and MV2+ obtained by the Taylor dispersion method and a possible explanation for this result is given.