The subsolidus phase relations of the BaOY2O3MnO2 system have been investigated in air. There are eight binary compounds, a new ternary compound and 11 three-phase regions in this system. The ternary compound with the BaO:YO1.5:MnO2 molar ratio of 8:2:5 was indexed by a rhombohedral lattice with a=5.7929 (6) and c=28.586 (4)angstrom. The synthesized compounds were determined to be stoichiometric except for the Ba1xYxMnO3 phase (x0.01).