A new trivalent bismuth ion conducting solid electrolyte, (Bi(x)Ge(1-x))(4/(4-x))Ta(PO(4))(3), was successfully developed by selecting the NASICON-type GeTa(PO(4))(3) as the mother solid. Although bismuth has two kinds of valence states of +3 and +5, it was clear that pure trivalent Bi(3+) ion conduction, without any electronic conduction by a valence change of bismuth in the oxygen pressure range over 10(-3) Pa, was realized by selecting the crystal structure and constituents of the solid. (C) 2010 Elsevier B.V. All rights reserved.