Nanosize effects have gradually become emphasized in the description of ion conduction properties of solids. The authors demonstrate a nanoionics effect that occurs in a proton-conducting perovskite upon dispersing fine platinum particles in the oxide. Both the ionic and hole conductivities are lost when the volume fraction of dispersed platinum exceeds a threshold value, resulting in an enormous reduction in both conductivities. These experimental results are discussed in accordance with a percolation model: the boundary layer, where charge carriers are suppressed by a space charge layer effect, blocks the bulk conduction of both protonic and electron-hole charge carriers. (c) 2007 The Electrochemical Society.