Electronic as well as ionic conducting properties for oxyapatite-type solid electrolytes based on lanthanum silicate, La 9.333 + x Si 6O 26 + 1.5 x (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 10Si 6O 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 n+ y )O 27-(2–0.5 n) y , improved its conductivity. On the other hand, replacement of the La-site with various ions for La 10(Si 6- y M n+ y )O 27-(2–0.5 n) y did little to improve conductivity. The electronic transport numbers for Al-doped OE-LSO with Fe-addition, (1- α){La 10(Si 5.8Al 0.2)O 26.9}- α(FeO γ ), evaluated with the Hebb–Wagner polarization method were very low: i.e., 1.1 × 10 − 3 and 2.9 × 10 − 3 under P(O 2) = 1.1 × 10 4 Pa at 1073 K for α = 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 5.8Al 0.2)O 26.9}–0.005(FeO γ ) 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.