Films of vinylidene fluoride - trifluoroethylene (VDF-TrFE ) copolymer were dehydrofluorinated in aqueous sodium hydroxide solutions by phase transfer catalysis(PTC). Conjugated C=C double bonds in the polymer back-bone were bonds in the dehydrofluorination of VDF-TrFE copolymer. The color of film became brown and finally black with rigid double bonds during the PTC reaction. It was found from UV/VIS spectroscopic measurements that the length of conjugated double bonds of dehydrofluorinated VDF-TrFE copolymer was longer than that of dehydrofluorinated poly(vinylidene fluoride) . It was ascribed to the reaction activity due to the difference of solubility parameter between the sample and reaction solution. Thermal behavior by DSC analysis showed that the dehydrofluorination occurred at least in the crystal surfaces as well as in the amorphous chains.