The frequency shifts andQchanges of a resonant microwave cavity were utilized as a basis for determining microwave susceptibilities and loss tangents for solids and liquids. The method employed consisted of varying the depth of penetration of a continuous cylindrical sample of the material into a cavity operating in the TM010mode with the resonant frequency and quality factor determined at each incremental setting of the perturbing sample. The perturbation of the cavity was achieved by advancing the sample into the cavity along the symmetry axis by employing a micrometer drive appropriately calibrated for depth of penetration of the sample. A differentiation method was used to obtain the half‐power points of the cavity resonance profile at each depth of penetration. The perturbation techniques for resonant cavities were used to reduce the experimental data obtained to physical parameters for the samples. The probing frequency employed was near 10 GHz.