The proton conducting 0.9MO(2)center dot 0.05In(2)O(3)center dot 1.3P(2)O(5) (M=Ti, Sn, Zr) electrolytes based on a core-shell structure were synthesized by a ball milling method. The core-shell type electrolytes showed the proton conductivities ranging from a higher value than those of Nafion membranes to 10(-5) Scm(-1) at intermediate temperatures of 150-200 degrees C, depending on the heat-treatment conditions. The samples with high conductivity were proved to adopt a core-shell structure by SEM observation, powder XRD analysis and P-31 MAS-NMR measurements. (C) 2008 Elsevier B.V. All rights reserved.