The dependence of the effective surface impedanceZeff=Reff+iXeffof superconducting thin films on the film thicknessd, on the magnetic field penetration depth &lgr;, and on the dielectric properties of the substrate material is investigated theoretically by means of impedance transformations. It was found that the effective surface resistanceReffcan be expressed byRSf(d/&lgr;)+RtranswhereRSis the intrinsic surface resistance of the superconductor. The functionf(d/&lgr;) describes the altered current density distribution in the film.Rtransarises from power transmission through the film. It depends ondand &lgr; as well as on the dielectric properties of the substrate material and is significantly altered in the case of a resonant background. The effective surface reactanceXeffof a superconducting thin film can be expressed byXS cosh(d/&lgr;) whereXS=&ohgr;&mgr;0&lgr; is the intrinsic surface reactance. Measurements ofZeffat 87 GHz have been performed for YBa2Cu3O7−&dgr;thin films grown epitaxially by laser ablation on SrTiO3, MgO, and LaAlO3. With the best films,Reff(77 K) values of 21 m&OHgr; andRS(77 K) values of 8 m&OHgr; were achieved. The temperature dependence of &lgr; was found to be in good agreement to both weak‐coupling BCS theory in the clean limit and the empirical two‐fluid model relation with &lgr; (0 K) values ranging from 140 to 170 nm and 205 to 250 nm, respectively.