The ion conductivity of a solid-state material is primarily a function of its chemical composition and crystal structure. However, interfaces can play an important role in the conduction process. The effects of interfaces on ionic properties can be controlled on the basis of "nanoionics". In this study, we demonstrate a nanoionics phenomenon observed in a blue platinum-containing perovskite. The proton conductivity changed reversibly in response to the precipitation of platinum nanoparticles and oxidation to form a solid solution. The results of XAFS measurements, TEM analysis, and electron holography provide evidence of the nanoionics phenomenon, and suggest a possible underlying mechanism for the conductivity change caused by the small amount of precipitated platinum nanoparticles in the perovskite oxide. (C) 2010 Elsevier B.V. All rights reserved.