A technique for measuring the dielectric constant and loss of isotropic films by mounting the specimen longitudinally at the center of a rectangular waveguide is proposed. Experiments have been performed at 9.375 GHz on the samples of Mylar and Teflon films ranging in thickness from 75 to 250 &mgr;m. The experimental results agree well (within 1–2%) with the accepted values. Analytical numerical results for typical samples are also given.