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The instrumentation, the design of the free-space focused microwave bridge system, and the technique for determining the complex permittivity of materials from ambient to 2500°C are described. The system employs a rotating disk sample which is located at the coincident focal point between two prolate spheroidal reflectors; the sample is heated on one side by oxyacetylene flames in such a manner that the microwave beam is not affected by the flames. Dynamic measurements are made of the sample insertion phase and insertion loss. A transient heat conduction analysis is used to determine the temperature profiles within the sample, and the electrical transmission data are correlated with the temperature profiles to give dielectric constant and loss tangent as functions of temperature. The relative dielectric constant and loss tangent of high purity slip-cast fused silica (density of 1.91 g/cm3) are determined to 2500°C. The relative dielectric constant vs temperature function is a segmented line and is 3.34 at ambient, increases linearly with temperature to 3.54 at 1500°C, 3.98 at 1600°C, 4.18 at 1700°C, and 4.32 at 2500°C. The loss tangent vs temperature function is also a segmented straight line when plotted on semilog paper with a value of 0.0005 at ambient and 0.09 at 2500°C.