Bioactive glasses are of considerable interest due to their potential for biomedical applications. In order to model the behaviour and dissolution of these materials it is important to have a good understanding of the local structure, network and the environment of the Ca and Na ions. Sodium is of particular importance since it is added to these materials specifically to decrease the network connectivity thereby increasing the dissolution of ions. It had been widely assumed Na was 6-fold coordinated in these systems but recent results have suggested the coordination number may be as low as four. This has profound consequences for our understanding of the connectivity of the glass network. We therefore propose to undertake a detailed study of these materials using neutron diffraction. A semi-isomorphic substitution experiment will be conducted using Na and Li Bioglass®.