We have proposed that homologous compounds of (ZnO) m In 2 O 3 with layer structures can become candidate materials for high-temperature thermoelectric conversion due to their low thermal conductivity and high electron mobility. Crystal structures can be modi5ed by the isoelectronic substitution of either divalent or trivalent metal ions for Zn or In ions, respectively. Substitution of Mg 2Ψ , Co 2Ψ , and Y 3Ψ gave rise to shrinkage of the c axis and elongation of the a axis of a hexagonal unit cell. Rietveld structure re5nement indicated that Mg 2Ψ and Co 2Ψ ions occupy both 3a and 6c sites, while Y 3Ψ ions occupy only 3a sites. An optimum amount of substitution of these cations increased electron mobility and hence thermoelectric e7ciency Z β«Ψβ¬ 2 / ( β«Ψβ¬ electrical conductivity, β«Ψβ¬ Seebeck coe7cient, β«Ψβ¬ thermal conductivity). Z values coupled with lowered thermal conductivity, which was possibly caused by suitable modi5cation of the electronic structure, were associated with distortion of the crystal structure. For instance, the 5gure of merit of (ZnO) 5 (In 0.97 Y 0.03 ) 2 O 3 was Z β«Ψβ¬ 1.3Ψ10 Ψ4 K Ψ1 .