This work deals with composites of high-temperature proton conductors and inorganic compounds such as carbonates, hydroxides, and chlorides. Discontinuities were found in the sT vs. 1/T plots which are ascribed to superionic phase transitions occurring in the interface region between the constituent phases. A similar superionic phase transition was also found between a composite of a proton conductor and a prominent oxygen ion conductor. The results provide a natural explanation for the high conductivities recently reported for similar composites of oxygen ion conductors and such inorganic compounds @B. Zhu, J. Power Sources, 114, 1 ~2003!# and also explains the high current densities obtained in intermediate temperature fuel cells based on such composites. © 2005 The Electrochemical Society. @DOI: 10.1149/1.1865592# All rights reserved.