The relationship between structure and conductivity has been investigated in the fast oxide ion-conducting system BINIVOX, Bi2NixV1-xO5.5-3x/2. (0.06 less than or equal to x less than or equal to 0.25) using X-ray powder diffraction and a.c. impedance spectroscopy. Arrhenius plots of conductivity in this system show two linear regions of different activation energy, one at low temperature and one at high temperature. The link between these two regions can take the form of a discontinuity or a simple but distinct change in slope. The particular behaviour adopted is composition dependent. (C)