Transient effect in fluorescence quenching in solution was measured and analyzed to study electron transfer reactions. In the analysis, the dependence of the rate constant on the donor-acceptor separation was taken into account on the basis of the Marcus equation. The diffusion equation with a distance-dependent sink term was solved numerically, and the solution was used to calculate the fluorescence decay curve. The convolution of the calculated decay curve and the instrument response function was fitted to the observed fluorescence decay curve in order to determine the electron transfer rate parameters involved in the Marcus equation. Fitting was made by a nonlinear least-squares method based on the Marquardt algorithm. It was found that the decay curves can be much better reproduced by this method than by the method based on the Collins-Kimball model. The rate parameters were determined for several donor-acceptor pairs in a viscous solvent, ethylene glycol, and compared with those determined by other methods.