The effect of fluorine incorporation on dielectric properties has been studied for a series of polyimides in order to distinguish between several contributing mechanisms to the generally observed decrease in the relative permittivity. Using low-frequency capacitance measurements after exhaustive in situ drying, in conjunction with refractive index measurements, the overall decrease in dielectric constant was semiquantitatively assigned between changes in the three modes of polarization via the use of fluorine/hydrogen and symmetric/unsymmetric analogs