Apatite-type lanthanide silicates/germanates have been attracting considerable interest as a new class of oxide ion conductors. The presence of interstitial oxide ions is vitally important for the high oxide ion conductivities of these phases, although the location of these interstitial sites has attracted some controversy. The accurate location of the interstitial oxide ions is non-trivial as their presence causes significant localised structural distortions. Therefore there is a need for further detailed studies in this area. Recently we have shown that Y doping in La9.33+xGe6O26+3x/2 increases the maximum interstitial oxygen content by over 50%, thus allowing greater ease of their location, and in this experiment a detailed structural study of these phases will be performed with a view to accurately locating these interstitial sites.