The aim of the present proposal is to use total scattering to investigate changes in the local structure of Ba3BiRu2O9. This compound undergoes an unusual magnetovolume transition at 176 K associated with the opening of a gap in the spin excitation spectrum of directly bonded Ru-Ru dimers within Ru2O9 bi-octahedra. In this experiment we wish to establish whether the transition is accompanied (or even driven) by a change in the local structure of the BiO6 octahedra that bridge Ru2O9 bi-octahedra. This could take the form of a valence state redistribution between the Bi and Ru cations, or of a charge disproportionation of the nominal Bi(IV) cations into Bi(III) and Bi(V).