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Free-space measurement techniques can be contactless and are able to accommodate large, flat sheets of dielectric material, making them useful for characterization of high-temperature, millimeter-wave, window and radome candidate materials. As part of the present work, a high-temperature, W-band (75–110 GHz), free-space measurement system was developed and used to characterize complex dielectric properties of bulk material samples at temperatures ranging from 25 °C to 600 °C. Two test cases, polyvinyl chloride (PVC) and CoorsTek 92% alumina, were measured at 25 °C and found to have $\boldsymbol {\epsilon }_{r}^\prime $ values of 2.731 ± 0.005 and 8.061 ± 0.027 at 95 GHz, respectively. The 25 °C PVC sample was measured to have a $\boldsymbol {\epsilon }_{r}^{\prime \prime }$ value of 0.032 ± 0.007. At 25 °C, the $\boldsymbol {\epsilon }_{r}^{\prime \prime }$ value of the 92% alumina sample was below the uncertainty threshold achievable with the present free-space measurement apparatus and could only be bounded to <0.009. As the alumina sample was heated to 600 °C, $\boldsymbol {\epsilon }_{r}^\prime $ and $\boldsymbol {\epsilon }_{r}^{\prime \prime }$ values increased to 8.501 ± 0.028 and 0.035 ± 0.008, respectively. The high-temperature behavior of the authors’ 92% alumina ceramic was found to be similar to that previously documented for Sumitomo AKP-50 alumina over the 25 °C–600 °C temperature range. In addition to the 92% alumina sample, three commercially available ceramic substrates (zirconium oxide, boron nitride, and silicon nitride) were also characterized at temperatures ranging from 25 °C to 600 °C. [ABSTRACT FROM AUTHOR]