Abstract: Dielectric constant κ′, loss κ″, and conductivity σ of single crystals of CoO, NiO, and mixed crystals of CoO–NiO as a function of frequency from 102 to 1010 cps and temperature from −193°C to 400°C have been determined. The dielectric constant κ′ is constant (12.9 at 25°C) in the whole frequency range in CoO. It decreases with frequency in NiO and mixed crystals CoO–NiO. In NiO κ′ reaches a constant value (11.9 at 25°C) at 105 cps, and in CoO·NiO (12.3 at 25°C) at 1010 cps. The dielectric constants extrapolated to 0°K are 10.9, 10.0, and 9.1 for CoO, CoO·NiO, and NiO, respectively. In all crystals the dielectric constant increases exponentially with temperature. Using modified Debye equations, two thermal activation energies for κ′ have been obtained: 0.042, 0.027, and 0.018 eV for temperatures below 40°C (frequency‐independent region); 0.33, 0.20, and 0.16 eV for temperatures above 40°C, for CoO, NiO, and CoO·NiO (frequency‐dependent region). The plot of log σ vs 1/T gives straight lines for CoO from 400° to 100°C, and NiO from 400° to 25°C with activation energies 0.73 and 0.66 eV. At lower temperatures the activation energies decrease continuously. The mixed crystals have two activation energies, one at high temperatures and the other at low temperatures. Variation of the activation energy with composition at high temperatures is less than that at low temperatures. The variation of activation energies in mixed crystals is correlated to lattice distortion and an increase of trivalent ion concentration.