Structure refinement was carried out for the powder X-ray diffraction data of A(3+)B(3+)O(3)-type perovskites, (La0.9Sr0.1) (MO3-delta)-O-III (M-III = Sc, In, and Lu) (LSMs) measured in wet air at 873 K. All the diffraction peaks of LSMs could be indexed as an orthorhombic GdFeO3-type structure (space group Pnma, No. 62). For the Sc-compound, the oxygen-oxygen (O-O) bond lengths of intra-ScO6 octahedra were shorter than those of inter-ScO6 ones, suggesting the predominant three-dimensional (3D) proton conduction along the edges of ScO6-octahedra. With increasing the size of B-site (M-III) cation from Sc3+ to Lu3+, the O-O bond lengths and tilting angles of BO6 ((MO6)-O-III) Octahedra increased. For the Lu-compound, the O-O bond lengths of intra-LuO6 octahedra were almost the same as those of the inter-LuO6 ones, suggesting the predominant one-dimensional (ID) proton conduction along O1-O1 and O2-O2 sites on the vertices of BO6 octahedra. This change in proton conduction pathway from 3D to I D would result in the lowering of proton mobility, hence in lower proton conductivity. (C) 2003 Elsevier B.V. All rights reserved.