Light‐induced enhancement of the third‐order nonlinearoptical susceptibility of a heterodimer consisting of two different porphyrin monomers, a zinctetra(4‐sulfonatophenyl)porphyrin and a zinctetra(4‐N‐methyl‐pyridyl)porphyrin, was observed when the sample was optically pumped by a 30 ps, 532 nm pump pulse. The temporal behavior of the enhancement was studied. It was found that the enhancement was due predominantly