We propose to use muon spin relaxation to examine the anomalous magnetic phase behaviour of Ba2MnMoO6. This compound shares the frustrated geometry that leads to a valence bond spin glass in Ba2YMoO6 but neutron diffraction shows long range antiferromagnetic order in Ba2MnMoO6 below 10 K. Remarkably, bulk magnetisation measurements provide no evidence of this magnetic transition.We request three days of beamtime on MuSR to measure 2 samples: Ba2MnMoO6 and Ba2Nd0.1Mn0.9MoO6. We will measure the muon response in both of these samples as a function of temperature and applied field. These experiments will allow us to resolve the contradictory evidence of bulk measurements and neutron scattering experiments and probe these materials for the existence of yet more exotic magnetic states, eg. spin liquid or valence bond spin glass states.