Polycrystals of Bi2K1-xNb2O8.5-delta (x = 0.0 - 0.5), which is a member (m = 2) of the bismuth layer-structured oxide (Bi2O2)(2+) (A(m-1)B(m)O(3m+1))(2-) and contains oxygen vacancies, were synthesized and their electrical conductivities were measured. By the Rietveld XRD analysis, a non-stoichiometric Bi2K0.65Nb2O8.325 was found to be a tetragonal phase and have the least amount of second phase in this system. Much higher oxide-ion conductivities were confirmed in Bi2K0.65Nb2O8.325 at low temperature region below 700 degrees C, compared with Bi2BaNb2O9 without extrinsic oxygen vacancies. Electronic conduction was observed at low oxygen partial pressures below 10(-4) MPa at 900 degrees C and 10(-5) MPa at 600 degrees C for Bi2K0.65Nb2O8.325.