Femtosecond dynamics of color changes due to photoinduced electron transfer and reverse reactions in polymeric 4,4′-bipyridinium di-iodides salts were studied with a fs laser pump–probe technique. The characteristic color change was shown to take place in less than 1 ps by fs-laser excitation of an ion-pair charge-transfer band, which may be applied to ultrafast THz all-optical switching devices using visible light. ©1998 American Institute of Physics.