We have synthesized the polycrystalline samples of the delafossite oxide CuCo1−yMgyO2 through the thermal decomposition of copper–cobalt–magnesium hydroxycarbonates. As is expected from the theoretical calculations, relatively large thermopower of S ∼ 700 μV K−1 is observed at room temperature in the parent compound CuCoO2, whereas the resistivity is too high even in the doped compounds compared with other thermoelectric oxides. The high-temperature transport is thermal-activation type characterized by two energy gaps, Δρ and ΔS, which are estimated from the resistivity and the thermopower, respectively. In the parent compound, we obtain Δρ ; 0.47 eV and ΔS ; 0.38 eV. We find that Δρ is larger than ΔS in all the samples, implying a mobility gap opening due to a grain-boundary scattering.