With low-loss, high thermal conductivity, and wide bandgap, single-crystal SiC, AlN, and diamond are promising materials for high-frequency, high-power electronics operating in extreme environments. However, precise knowledge of their permittivity at high frequencies remains limited. To address this gap, this work presents a comprehensive characterization of complex permittivity in single-crystal AlN and diamond from microwave to millimeter-wave frequencies using high-quality (Q ≥ 105) Fabry-Perot resonators, extending a measurement technique validated on single-crystal SiC. For 2″ single-crystal AlN, the measured εR is constant at 7.805 ± 0.007, while the tanδ increases linearly with frequency on the order of 10-4, remaining below that of fused silica. A 93-mm-diameter AlN wafer confirms scalability and reproducibility. For single-crystal diamond (≥ 1″), the εR is frequency-independent, while the tanδ increases with decreasing frequency, suggesting the influence of defects such as N-V centers. Together, these results confirm both materials’ suitability for high-power, high-frequency applications in harsh environments.