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X-ray diffraction and capacitance measurements have been employed to study the structural and dielectric behavior of the ferroelectric-paraelectric phase transition under the influence of a large external electric field. The samples under study are ultrathin (15–100 nm) Langmuir–Blodgett films of a copolymer of vinylidene fluoride (70%) with trifluoroethylene (30%) deposited on aluminum-coated silicon. In situ θ-2θ x-ray diffraction was used to measure the change in interlayer spacing perpendicular to the film surface, corresponding to the (110) direction and indicating that the polymer chains along (001) lie predominantly in the plane, while capacitance measurements were used to monitor the behavior of the dielectric constant of the film. Application of a large electric field, up to 265 MVm, raises the phase transition temperature and can convert the nonpolar trans-gauche paraelectric phase to the polar all-trans ferroelectric phase in a reversible manner.