The dielectric properties of titanium-doped magnesium oxide (Ti/MgO) and nickel-doped magnesium oxide (Ni/MgO) single crystals have been measured in the range of temperature from 300 to 450 K at the microwave frequency of 9.31 GHz. For both crystals the dielectric properties are found similar. From the conductivity data, the activation energy in the measured temperature region has been estimated to be 0.15 eV. The values of the temperature dependence (e′−1)−1(e′+2)−1 (∂e′/∂T)p have been calculated. The data confirms the Bosmann and Havinga postulate that, for materials in which the dielectric constant;e′ is less than 20 the temperature dependence should be positive.