The kinetics of the intermolecular electron transfer between pyrene and pyromelitic dianhydride is studied by two experimental technigues : transient absorption measurements and degenerate four-wave mixing after optical excitation by picosecond light pulses. The observed time evolution of the electron transfer reaction depends strongly on the wavelength of the pump pulse. Excitation of pyrene leads to a diffusion-controlled electron transfer (in agreement with theoretical calculations), while excitation of the ground state donor-acceptor complex shows a different time behavior. Comparison of the decay times and the molecular hyperpolarizabilities provides clear evidence for two different excited charge transfer complexes.