The In₂O₃-SnO₂-ZnO system is important for the transparent conducting oxides. The isothermal phase diagrams of the ternary In₂O₃-SnO₂-ZnO system at 1400 and 1500 °C were experimentally studied by quenching experiments followed by XRD and EPMA phase analysis. A new ternary In₂Sn₂Zn₂O₉ phase with crystal structure analogous to R3c ZnSnO₃ was found, and its eutectoid decomposition reaction to In₂O₃ bixbyite s.s. + Zn₂SnO₄ spinel s.s. + SnO₂ was determined to be 1310 ± 10 °C. In addition, a small homogeneity range of this ternary phase toward In₂O₃ was found: In_2+2xSn_2-xZn_2-xO₉ where x = 0.0 ∼ 0.2 by Zn²⁺ + Sn⁴⁺ = 2 In³⁺ substitution.