Langmuir-Blodgett (LB) films and ultrathin polymer films containing 4,4’-bipyridinium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-) were deposited on the surface of an optical waveguide (OWG) glass. Colour changes due to photoinduced electron transfer reaction upon excitation of ion-pair charge-transfer complexes between 4,4’-bipyridinium and TFPB- in these ultrathin films, even in a single monolayer, were sensitively detected by the OWG technique in a degassed atmosphere. The LB and polymer films showed different time-dependent photoresponses.