Abstract The 4,4′-bipyridinium ions formed ion-pair charge-transfer (CT) complexes with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in organic solutions, microcrystalline films, Langmuir-Blodgett films, and polymer films. Reversible colour changes between pale yellow and blue due to photoinduced electron transfer upon excitation of an ion-pair CT band and thermal reverse reactions were observed, which we call the photoinduced electrochromism. The rate of reverse reactions markedly depended on temperature and microenvironments. The blue-coloured state was formed very rapidly by pulsed laser excitation, which was stored without decay below 0°C in polymer films. Such photoinduced electrochromism may be applied to a new high-speed and high-density optical memory.