We describe a time-domain reflectometry (TDR) technique to measure the dielectric permittivity of film materials with enhanced dielectric constant. The test specimen consists of a planar capacitor terminating coaxial waveguide. The complex permittivity is obtained from analysis of the incident and the reflected voltage waves. In order to improve accuracy at higher frequencies, we introduced a propagation term correcting the lumped element model. The applicability of the method has been verified at frequencies from 100 MHz to 10 GHz on several polymer composite films, 8 mum to 100 mum thick, having a dielectric constant ranging from 3.5 to 40. When compared to other techniques, TDR provides a more intuitive and direct insight into capacitance density and impedance characteristics, which is useful in high-frequency characterization of materials for embedded capacitance applications