We present mm-wave transmission as a complementary technique to characterize the properties of superconducting thin films, obtaining information on both the real and imaginary part of the dielectric function. Examples are given to illustrate the strength of the technique to measure the response of dielectric materials and stratified systems. We demonstrate the BCS-behavior of a conventional superconductor NbN, and show furthermore the unconventional behavior in both the absorptive and reactive part of the conductivity of a 20 nm thick DyBa2Cu3O7-delta film