ZnO doped with In₂O₃ is a promising n-type thermoelectric (TE). It is well known that addition of In₂O₃ results in formation of multiple structural defects in the wurtzite structure of ZnO, modulated in a way that they could be considered as homologous (ZnO)_k In₂O₃ phases. Basically two types of defects are present which primarily affect the TE characteristics: planar InO₂^- defects which induce electrical conductivity and “zig-zag” type ZnO/In defects in between planar ones causing phonon scattering for reduced thermal conductivity. We studied phase formation, microstructure development and TE characteristics in compositions (ZnO)_kIn₂O₃ with k values from 5 to 36 in the temperature range from 900°C to 1400°C. It was shown that densification and grain growth follow formation of the Zn_kIn₂O_k+3 phases and that for higher k values higher firing temperature and longer times are required to obtain ordered phase of that k value. Samples sintered at 1300^oC for 22 hours had ZT (950K) values of 0.11 for k=5 and it dropped to about half for the samples with k=11 and 18. Furthermore, the effect of partial substitutions of In³⁺ with Al³⁺, Y³⁺ and Mn⁴⁺ in compositions (ZnO)_k(In₂O₃)_1-xM_x  for k= 5, 11 and x=0.01, 0.03, 0.05, 0.1 and 0.2 on characteristics of the samples sintered at 1500^oC for 2 hours was studied. The thermoelectric characteristics will be discussed in regard to the samples composition, microstructure, structure and charge carrier concentration determined by the Hall measurements.