The compound CsH2PO4 has emerged as a viable electrolyte for intermediate temperature (200-300 C) fuel cells. With sufficient humidifcation to suppress dehydration, a transition to a high conductivity (superprotonic) phase around 230 C is observed. The complete phase behavior in the CsH2PO4-CsPO3-H2O system has been recently elucidated [5] and provides a phase map that enables accurate study of the properties of the superprotonic, cubic phase of CsH2PO4. While the phase behavior is now well-characterized, many critical questions regarding the connection between structure and proton transport remain open. This proposal aims to answer three questions: What is the connection, if any, between hydrogen bond disorder and phosphate disorder at high temperature? How do these factors contribute, if at all, to the high proton conductivity? Are there changes in the structure of the monoclinic phase on approaching the cubic phase that enable or presage the high conductivity phase? The answers to these questions will be essential to the design of next generation materials with even more attractive properties.