Cubic alpha-ZrW2O8 is the classic example of a material exhibiting the unusual property of negative thermal expansion (NTE). Despite this effect having been discovered 20 years ago, it is not well understood. NTE is widely thought to arise due to correlated libration of large rigid structures, but the details are far from clear. Recent theoretical work has concentrated on determining which phonon modes contribute most to NTE. We have already obtained inelastic neutron scattering data in an attempt to test these calculations. However, we discovered that an unknown fraction of our sample contains the metastable orthorhombic gamma phase of ZrW2O8, which displays much weaker NTE. We wish to characterise the phase fractions in our samples, which will allow us to tell if our INS data can be easily analysed. We will also heat-treat the samples in-situ to recover phase-pure alpha ZrW2O8