The (ZnO)_kIn₂O₃ system is interesting for applications in the fields of thermoelectrics and opto-electronics. In this study we resolve the complex homologous phase evolution with increasing temperature in polycrystalline ceramics for k = 5, 11 and 18 and its influence on the microstructural development and thermoelectric properties. The phase formation at temperatures above 1000 °C is influenced by the local ZnO-to-In₂O₃ ratio in the starting-powder mixture. While the Zn₅In₂O₈ equilibrium phase for k = 5 is formed directly after sintering at 1200 °C, the formation of the k = 11 and k = 18 equilibrium phases proceeds at higher temperatures by diffusion between the initially formed phases, the lower k Zn₅In₂O₈/Zn₇In₂O₁₀ and the higher k Zn_kIn₂O_k+3 (9 < k < ∞). Such phase formation affects the sintering and grain growth, and consequently, with the degree of structural and compositional homogeneity, also the thermoelectric characteristics of the (ZnO)_kIn₂O₃ ceramics.