Ho2O3 and Tm2O3 doped Bi2O3 composite electrolyte type materials for solid oxide fuel cells (SOFCs) operating at intermediate-temperature were investigated. The bismuth-based ceramic powders were produced by using conventional solid-state synthesis techniques. The products were characterized by means of scanning electron microscopy (SEM), X-ray powder diffraction (XRD), differential thermal analysis/thermal gravimetry (DTA/TG), and the four-point probe technique (4PPT). XRD and DTA/TG measurements indicate that all of the samples have the stable fluorite type face centered cubic (fcc) delta-phase. 4PPT measurements were performed in the temperature range 150-1000 degrees C in air and these measurements showed that the electrical conductivity of the samples decrease with increasing amount of Tm2O3. This increase in the electrical conductivity of the samples could be attributed to the increase in the numbers of highly polarizable cations and oxide ion vacancies. The highest conductivity value was found as 5.31 x 10(-1) Omega cm(-1) for the (Bi2O3)(1-x-y)(Ho2O3)(x)(Tm2O3)(y) ternary system (for x=20 and y=5 mol%) at 1000 degrees C. The activation energies of the samples were calculated from log a graphics versus 1000/T. These calculated results showed that the translation motion of the charge carriers, oxygen vacancies, and space charge polarizations are responsible for the change in activation energy as a function of temperature. (C) 2012 Elsevier Ltd. and Techna Group S.r.l. All rights reserved.