We report on the microwave properties of Nd0.67Sr0.33MnO3−x (NSMO) and NSMO/YBa2Cu3O7−x thin film heterostructures. The quality factor (Q) and center frequency of a 13 GHz shielded dielectric cavity resonator, with the film comprising one surface, were measured as functions of temperature and direct current magnetic field. The Q versus field data was theoretically simulated using the Landau–Lifschitz–Gilbert dynamic permeability equation thus demonstrating that the microwave losses are determined by the ferromagnetic properties of the films. Our results indicate that the field-dependent permeability μ(B) of NSMO films holds the potential to create magnetically tunable microwave devices. © 1999 American Institute of Physics.