Single crystals of Sr-bearing neodymium silicates with the oxyapatite structure were grown by the floating zone method. The grown crystals were free from any macroscopic defects and their quality was sufficient for electrical measurements. Electrical conductivity was enhanced by about three orders of magnitude by the introduction of cation vacancies to Sr2Nd8(SiO4)(6)O-2, a compound without cation or oxygen ion vacancies. The introduction of oxygen ion vacancies to Sr2Nd8(SiO4)(6)O-2 and Nd-9.33(SiO4)(6)O-2 did not significantly affect their electrical conductivity. Cation vacancies may play an important role for oxygen ion conduction in oxyapatite type rare-earth silicates. (C) 2004 Elsevier B.V. All rights reserved.