A series of apatite-type La-Ge-O ceramics were prepared and their cation-defect at the 4f+6h sites and oxide ion-defect at 2a site were investigated. In LaxGe6O12+1.5x ceramics of x = 6-12, the higher conductivities were obtained in the region of apatite composition, La-x(GeO4)(6)O1.5x-12 (x=8-9.33), and the highest conductivity was achieved for La-9(GeO4)(6)O-1.5 (x=9), where the number of cation (La3+) occupying the 4f+6h sites is 9 and the number of oxide ion occupying the 2a site is 1.5. The ceramics with cation- and oxide ion-defects were La9-0.66xSrx (GeO4)(6)O-1.5 (x=0-1), La9-1.33xZrx (GeO4)(6)O-1.5 (x=0-3), La9-xSrx(GeO (4))(6)O1.5-0.5x (x=0-3), La9-xZrx (GeO4)(6)O1.5+0.5x (x=0-1), La-x(GeO4)(3x-21)(AsO4)(27-3x)O-1.5 (x=0-3), La-x(GeO4)(33-3xx)(AlO4)(3x-27)O-1.5 (x=0-3), La-9(GeO4)(6-x) (AlO4)(x)O1.5-0.5x (x=0-3), La-9(GeO4)(6-x)-(AsO4)(x)O1.5+0.5x (x=0-1), La9.33-xSrx (GeO4)(6)O2-0.5x (x=0-1.2) and La-x(GeO4)(4.5)(AlO4)(1.5)O1.5x-12.75 (x=8.8-9.83), which were prepared by the partial substitution of La3+ and G(4)(4-) tire basic apatite La-9(GeO4)(6)O-1.5 with Sr2+ or Zr4+ and AlO45- or AsO43- Such substitutions lowered the conductivity of La-9(GeO4)(6)O-1.5. These results were discussed by the electrostatic interaction between Sr2+, Zr4+, AlO45- or AsO43- and oxide ion as a conductive species. (C) 2004 Elsevier B.V. All rights reserved.