We have studied the formation, polymorphism, and electrical conductivity of Bi2O3-based compounds with the general formula (Bi2O3)x(Nd2O3)y(MoO3)z (x + y + z = 1) in the ternary system Bi2O3–Nd2O3–MoO3. It has been shown that the system contains Bi2O3-based solid solutions with cubic, tetragonal, monoclinic, and rhombohedral structures, depending on composition. Bi2O3-based compounds with the cubic fluorite structure have been identified in two broad composition regions: 0.56 ≤ x ≤ 0.9, 0.03 ≤ y ≤ 0.22, 0.03 ≤ z ≤ 0.23 and 0.36 ≤ x ≤ 0.6, 0.25 ≤ y ≤ 0.47, 0 ≤ z ≤ 0.21. In addition, a large region of tetragonal compounds has been detected, with 0.37 ≤ x ≤ 0.56, 0.19 ≤ y ≤ 0.38, and 0.23 ≤ z ≤ 0.33. The conductivity of the samples studied increases with Bi2O3 concentration: from 0.01 ((Bi2O3)0.36(Nd2O3)0.45(MoO3)0.19 sample) to 0.7 S/cm ((Bi2O3)0.9(Nd2O3)0.07(MoO3)0.03 sample) at 800 °C. The cubic and tetragonal symmetries of the samples persist after rapid heating and cooling, but prolonged annealing in a limited temperature range can lower the symmetry of some samples. Heating the samples to above their instability range restores their original structure and properties. In addition, we have identified (Bi2O3)x(Nd2O3)y(MoO3)z compounds that remain stable, without symmetry changes, at any temperature, even at long annealing times. Display omitted •Wide stability fields of Bi2O3-based phases exist in Bi2O3–Nd2O3–MoO3 system.•Cubic, tetragonal, monoclinic, and rhombohedral compounds were found.•Some compounds may decrease their symmetry in a limited temperature range.•Some compounds remain stable at any temperatures and annealing time.•The conductivity rises with increase of Bi2O3 concentration.