There has been a renewal of interest in lithium ion conducting solid electrolytes for use in lithium batteries. The ease of handling and scale-up, make oxide based systems, most industrially relevant in this respect. The present proposal seeks to use the advantages of neutron diffraction to examine the defect structure in two promising electrolyte systems with the LISICON structure using a combination of high resolution powder neutron diffraction and reverse Monte Carlo Modelling of high-flux neutron total scattering data. Details of the lithium ion distribution in lithium-rich LISICON analogues will be used to propose models of the conduction mechanism in these fast ion conductors.