Low-temperature vacuum reduction was used for the preparation of the oxygen-deficient BiMnO2.81 sample in a bulk form from stoichiometric BiMnO3. The transformation occurs in vacuum better than 10(-3) Pa at a narrow temperature range of 570-600 K. The structure of the new phase was analyzed using synchrotron X-ray powder diffraction data. BiMnO2.81 crystallizes in a perovskite-type cubic structure (space group I-43d) with a = 15.88552(S) angstrom corresponding to a 4a(p) superstructure, where a(p) is the parameter of the cubic perovskite subcell. Oxygen vacancies are ordered, and one oxygen site in BiMnO2.81 is completely vacant, resulting in MnOs pyramids. BiMnO2.81 is rather unstable in air and slowly restores its oxygen content even at room temperature.