Effects of dopant content and synthesis temperature on phase relations and conducting behavior of lanthanum silicate, La9.33+xSi6O26+1.5x (LSO) electrolytes were investigated. The La-excess-type Fe-doped LSO, La-10(Si5.5Fe0.5)O-26.75 synthesized at 1973 K showed lower grain boundary as well as grain interior resistances, and then, had high total conductivity (6.9 x 10(-3) S cm(-1) at 873 K and 3.6 x 10(-5) S cm(-1) at 573 K). In addition, the chemical instability owing to a high lanthanum activity in the La-excess-type LSO (x = 0.67) specimen could be eliminated by incorporating iron into the specimen. The synchrotron radiation microbeam X-ray diffraction technique revealed that this specimen contains a small amount of new secondary phase, which differs from the situation found in a highly conductive but chemically unstable Al-doped LSO specimen with the same x value and the same synthesis temperature.