MIS heterostructures have been fabricated with a 56 nm film of ferroelectric polymer as the insulating layer. High frequency C-V measurements have been made at room temperature, for bias voltages from -5 V to +5 V. A shift of the observed C-V curve from the theoretical curve indicates the presence of fixed positive charges, with density of 0·52 mC m−2, in the polymer layer. Injected and polarisation charges also shift the curves, depending on the direction and sweep rate of the bias voltage. At bias voltages above +1 V dipole movement in the polymer reduces the shielding of inversion layers from depletion region, leading to a widening of the depletion region and a consequent decrease in capacitance.