The synthesis route for single phase BiMnO 3 from Bi 2 O 3 and Mn 2 O 3 has been undertaken, in situ, in a diamond anvil cell. The starting powders were mixed in a stoichiometric ratio and loaded into a diamond anvil cell with a gasket aperture of 150μm. A ruby chip was used to measure the pressure and a circular resistive heater surrounded the central core of the cell. The reaction was studied as a function of pressure and temperature to 6.7GPa and 480°C by X-ray diffraction. There was evidence of the onset of a reaction between the starting oxides at 314°C and 6.7GPa but there was no evidence of transformation of the monoclinic Bi 2 O 3 to the cubic form before the reaction began. A monoclinic phase related to the structure of multiferroic BiMnO 3 was observed to form with space group Cc, a=10.21(1), b=5.356(6), c=10.38(1)Å and β=116.87(6)°. A good fit for this structure was obtained by modelling a distorted BiMnO 2 unit cell with disordered Bi and Mn sites. We did not go sufficiently high in temperature to observe the formation of single phase BiMnO 3 but the precursor phase was observed to persist when returned to room temperature and pressure.