A Sr-3(Ca1+xNb2-x)O9-delta High Temperature Protonic Conductor has been produced using the melt growth process with the aim of reducing the grain-boundary blocking effect in proton conduction. The microstructure is characteristic of a cellular growth with cell dimension of 10-20 mu m in width and 100 mu m in length. The cells have distinct core and shell regions. A composition gradient in bivalent to pentavalent cation ratio is observed from the core to the shell. The cores act as channels for hydrogen diffusion. Nano-scaled domains have been revealed inside the cells identified as domains of 1:1 order of the cations on the B sites and orientation variants in the oxygen octahedra tilting. Ion Beam Analyses using a Nuclear Microprobe reveal only a slight hydrolysis of the surface after heat treatment of 10 h at 500 degrees C in wet air. Protonic conductivity is discussed and improvements are suggested by optimizing the calcium to niobium ratio of the constituent elements and designing larger cells. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.