Electronic as well as ionic conducting properties for oxyapatite-type solid electrolytes based on lanthanum silicate, La(9.333) (+) (x)Si(6)O(26 + 1.5x) (LSO) were investigated in the oxygen-excess region (x > ca. 0.3). We have found that the oxygen excess-type LSO (OE-LSO), namely La(10)Si(6)O(27) on weighted basis, exhibited high conductivity, and substitution of the Si-site of LSO with some dopants (M(n+)) had a positive effect toward the conducting property. Furthermore, it was also found that addition of a very small amount of iron ions into the M-doped OE-LSO, La(10)(Si(6-y)M(y)(n+))O(27-(2-0.5n)y), improved its conductivity. On the other hand, replacement of the La-site with various ions for La(10)(Si(6-y)M(y)(n+))O(27-(2-0.5n)y) did little to improve conductivity. The electronic transport numbers for Al-doped OE-LSO with Fe-addition, (1-alpha)(La(10)(Si(5.8)Al(0.2))O(26.9))-alpha(FeO(gamma)), evaluated with the Hebb-Wagner polarization method were very low: i.e., 1.1 x 10(-3) and 2.9 x 10(-3) under P(O(2))=1.1 x 10(4) Pa at 1073 K for alpha = 0.00 and 0.005, respectively. Conductivity for each sample was unchanged under humidified atmosphere at 1073 K sustained for over 50 h, revealing that both compositions were chemically stable. It was concluded that 0.995{La(10)(Si(5.8)Al(0.2))O(26.9)}-0.005(FeO(gamma)) is suitable for the fuel cell electrolytes because of its high and almost pure ionic conductivity, and its good chemical stability under humidified as well as reducing conditions. (C) 2010 Elsevier B.V. All rights reserved.