Durable ceramic waste forms that incorporate a wide range of radionuclides have the potential to broaden the available disposal options and to lower the storage and disposal costs associated with advanced nuclear fuel cycles.  Ceramic waste forms have also been studied because of their superior chemical durability and the ability to incorporate a broad spectrum of chemical species within the lattices. Recent work has shown that they can be produced from a melting and crystallization process similar to melter technology currently in use for High Level Waste (HLW) vitrification in several countries around the world.   Cesium (primarily Cs¹³⁷) is one of the more problematic fission product radionuclides to immobilize because of its volatility at high temperature and its tendency to form water-soluble compounds.  Hollandite oxides of the form Ba_xCs_yGa_2x+yTi_8-2x-yO₁₆ (x=1.33, 1.04, 0.667, 0; y=0, 0.24, 0.667, 1.33), similar to materials used as Li battery cathodes are currently under evaluation for the immobilization of Cs.  The phase formation, experimentally determined ionic conductivity, and propensity for elemental release were determined for substituted hollandites containing different levels of Cs. Implications for controlling the ionic transport in diverse applications of ceramic-ceramic composites ranging from mixed ionic and electronic conducting separation membranes to nuclear waste immobilization will be discussed.