Apatite-type lanthanum silicates doped with different multivalent cations at the Si site, La10Si5.5B0.5O27 + delta (B = In, Si, Sn, Nb) ceramics, are synthesized via the high temperature solid state reaction process. X-ray diffraction and complex impedance analysis are used to analyze crystal structure and electrical conductivity of La10Si5.5B0.5O27 + delta ceramics. Both La10Si5.5In0.5O26.75 and La10Si5.5Nb0.5O27.25 ceramics consist only of a hexagonal apatite structure with a space group P63/m, however, La10Si6O27 and La10Si5.5Sn0.5O27 ceramics are composed of a hexagonal apatite structure and a small amount of second phase La2SiO5. The measured total conductivity of each composition gradually increases with increasing temperature from 673 K to 1173 K, which follows the Arrhenius behavior. Lanthanum silicates doped with trivalent In3+ or pentavalent Nb5+ cations at the Si4+ site have a higher total conductivity than undoped lanthanum silicate, however. La10Si5.5Sn0.5O27 ceramic doped with tetravalent Sn4+ cations at the Si4+ site exhibits a comparable total conductivity to undoped La10Si6O27 ceramic. La10Si5.5In0.5O26.75 and La10Si5.5Nb0.5O27.25 ceramics have a total conductivity of 1.75 x 10(-2) S.cm(-1) and 1.23 x 10(-2) S.cm(-1) at 1073 K, respectively, which are considerably higher than that of undoped La10Si6O27. At temperature of 773 K, La10Si5.5In0.5O26.75 ceramic has the highest total conductivity of 8.92 x 10(-4) S.cm(-1). (c) Elsevier B.V. All rights reserved.