Plasma copolymerization is utilized to fabricate thin photonic films based on hexamethyldisiloxane (HMDSO, C 6 H 18 Si 2 O) and octafluorocyclobutane (OFCB, C 4 F 8 ). The structure of the plasma copolymerized films is examined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR), and the optical properties including refractive index, n, and extinction coefficient, k, are determined by spectroscopic ellipsometry. The refractive indices, which range from 1.38 to 1.54, can be manipulated by adjusting the volume ratio of the two monomers, HMDSO and OFCB, during the polymerization. A nonlinear relationship between the refractive index and extinction coefficient with comonomer feed ratio is observed. A strong initial increase in both n and k as the amount of HMDSO is increased is attributed to significant defluorination of the resultant polymer films coupled with the formation of a C-C rich crosslinked network. Once the network incorporates substantial amounts of Si-C and Si-O bonds, the refractive index starts to decrease slowly due to a lowering of the density. XPS and FTIR results confirm the changes in internal structure consistent with this mechanism.