Colour changes due to photoinduced electron transfer reaction were studied by picosecond pulsed laser in thin polymer films containing 4,4’-bipyridinium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate salts as part of the main chain at 98 - 300 degrees K. The transient absorption at about 600 nm appeared in less than 20 ps upon excitation of ion-pair charge-transfer absorption, which was controlled by the laser pulse width. The decay curve was analyzed by a fast component with a fraction of about 0.2 - 0.3 and lifetime of a few hundreds ps together with an extremely slow one corresponding to steady and reversible colour changes. The lifetime of the former slightly increased with decreasing temperatures. A reaction mechanism is proposed based on these results.