The fluorescence behaviour of poly(ethylene terephthalate) (PET) films prepared by using a spin-casting method was measured. The wavelength of the fluorescence peak was found to be dependent on the casting temperature, T-cast. This fluorescence is neither from a ground state dimer nor from an excimer. It is the redshifted fluorescence of either the phenylene group or the phenylene exciplex due to interactions involving some contributions of charge-transfer stabilization among phenylene and carbonyl groups and/or phenylene groups. Both d.s.c. and i.r. measurements clearly showed that the fluorescence reflects whether the main-chain phenylene ring of PET is in the crystal region or in the amorphous region.