A study of ionic transference numbers in fluorite-based doped bismuth oxides of general formula Bi3.5Nb1-xYxO7.75-x (0.0 <= x <= 0.8) measured using a modified EMF method is presented. The modified method appears to yield transference numbers that are more consistent with the observed behaviour in a.c. impedance spectra compared to classical EMF methods. The results show that niobium-rich compositions have a significant electronic contribution to total conductivity at lower temperatures, but that at elevated temperatures above ca. 600 degrees C the electronic contribution is negligible. Yttrium doping yields almost pure ionic conductors at all temperatures studied. This is explained with reference to structural features in terms of a disruption of electronic conduction pathways. (C) 2009 Elsevier B.V. All rights reserved.