ReO3 is a well-known oxide framework with unusual thermal expansion behaviour, exhibiting multiple regions of both positive and negative thermal expansion (NTE). We propose to investigate its local structure using total neutron scattering. We will analyse the resulting data using the reverse Monte Carlo method, with an emphasis on understanding the rigid unit mode fluctuations associated with NTE. The analysis will construct the distribution of atomic positions associated with these fluctuations, and from its temperature dependence we will be able to observe the effects of the anharmonic effects that must be the physical origin of the anomalous NTE behaviour. Comparison with ongoing analysis of the isomorphic material ScF3 will give new insights into the behaviour of these archetypal NTE materials.