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With the increasing use of ferrites in microwave integrated circuits, it becomes important to determine the temperature stability of the effective values of dielectric constant and unmagnetized permeability of ferrites in this application. This paper reports the results of such measurements on several ferrites. The test samples were slabs 2 × 0.5 × 0.032 in. In order to provide a basis for understanding the intrinsic properties of the ferrites, the slabs were tested first by completely copper plating the sample except for an iris at each end. The resonant frequencies of the samples were measured at room temperature and at an elevated temperature, first without a bias field, then with a bias field. From the measurements, data were obtained on the intrinsic dielectric constant and the unmagnetized permeability, and their temperature coefficients. Formulas are given to calculate the effective values from the intrinsic values. The original plating was then removed and the sample replated as a microstrip line. The magnetic filling factor was obtained from measurements on the microstrip line.