Highly dense pellets of an oxygen excess-type lanthanum silicate (La9.333+xSi6O26+1.5x, x > ca. 0.3, OE-LSO) were successfully fabricated, and their electrical conducting properties were studied. The replacement of Si by Al enhanced its conductivity, and the slightly Al-doped OE-LSO specimen [La-9.62(Si5.79Al0.21)O-26.33] had excellent features as a solid electrolyte; that is, it had high ionic conductivity and was highly stable under reducing as well as oxidizing conditions at 873-1073 K. In addition, the ionic transference number was higher than 0.99. In the fuel cell utilizing this electrolyte (0.72 mm thick), (La0.6Sr0.4)(Co0.2Fe0.8)O3-delta cathode, and Ni-Ce0.9Gd0.1O1.95-delta anode, good performance with the maximum power density of ca. 0.25 W cm(-2) was obtained at 1073 K. In addition, this electrolyte also had high compatibility with these conventional mixed conducting electrodes, according to an analysis near the electrode/electrolyte interfaces after the fuel cell test. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3464803] All rights reserved.