Additions of a small amount of alumina to bioactive phosphate-based glass significantly improves the formation of the hydroxycarbonate apatite layer on the surface when exposed to simulated body fluid. The reason for this increased bioactivity is believed to be structural with the alumina reducing the solubility of the glass. Concurrent with this increased stability, there is also an increase in bulk density. However, above 5 mol% Al2O3 both the density and bioactivity start to decrease. These observations suggest that a structural rearrangement occurs at this composition. Here we propose to probe this structural change using neutron diffraction. Our results will be combined with those from 31P and 27Al MAS NMR to produce an accurate structural picture of the material that can be related to the observed changes in its properties at this critical composition.