Fluorine atom incorporation into polymers typically yields a low refractive index and dielectric constant. Two effects contribute to this: the decreased electronic polarizability of the C-F bond relative to C-H, and the introduction of additional fractional free volume due to less dense polymer chain packing. Here, it is reported that the change in fractional free volume contributes an average of 49% of the observed decrease in refractive index and 64% of the 1 kHz dielectric constant for symmetric structures. This surprisingly large magnitude is found comparing exact fluorine/hydrogen analogous pairs of polyimides using positron annihilation lifetime (PAL) spectroscopy and the application of a Lorenz-Lorentz normalization. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.