Neutron powder thermodiffractometry has been used to study the hst oxygen ion conductor Bi-4(V0.85CO0.15)(2)O11-delta from 1.6 to 850 K. The following phenomena are observed. (i) An order-disorder transition appears at high temperature (near 780 K), corresponding to the reversible growth of ordered domains in a partially disordered lattice. The sharp drop in satellite reflection intensities confirms the first-order nature of this transition which can be associated with the decrease in activation energy observed on the conductivity curves. (ii) At the order-disorder transition, the unit cell is expanded along the a- and b-axes, contracted along the c-axis and the mean-square atomic displacements increase more rapidly. (iii) In the low temperature range (1.6-250 K), the evolution of the mean-square atomic displacements is described using the Debye and Einstein models. Fitting these models brings to light an anomaly in the lattice vibration behaviour below an average temperature of 200 K, which can be explained as the freezing of the anionic diffusion below this characteristic temperature.