Fourteen kinds of alkali-metal rare earth germanate ceramics, MREGeO4 (M = Li, Na, K; RE = rare earth), were synthesized and their electrical properties were investigated. The crystal structures of the major phases of MREGeO4 could be classified into four groups; hexagonal (apatite type), tetragonal, orthorhombic (olivine type) and orthorhombic (beta-K2SO4 type). The tetragonal group exhibited higher ionic conductivity than the other groups in the range 300 to 700degreesC. The highest conductivity was achieved for the LiNdGeO4 having a tetragonal structure (2.95 x 10(-2) S cm(-1) at 700degreesC). Analyses on the basis of the Nernst's equation indicated that the two electron reaction associated with carbon dioxide molecule takes place at the detection electrode above 400degreesC regardless of the kind of electrolyte. The electromotive force, EMF, of the potentiometric CO2 gas sensors designed using MLaGeO4 (M = Li, Na, K) increased linearly with an increase in the logarithmic value Of CO2 partial pressure, in accordance with the Nernst equation. (C) 2005 Springer Science + Business Media, Inc.