The oxide ionic conductor Ba3W1.2Nb0.8O8.6 has been synthesized as part of an investigation into the new class of Ba3M'M '' O-8.5 (M' = W, Mo; M '' = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W6+ for Nb5+ in Ba(3)W(1+x)Nb(1-x)O(8.5+x/2 )leads to an increase in the oxygen content, which enhances the low-temperature ionic conductivity. However, at 400 degrees C, the ionic conductivity of Ba3W0.2Nb0.8O8.6 is still significantly lower than the molybdenum compound Ba3MoNbO8.5. Remarkably, at 600 degrees C the bulk oxide ionic conductivities of Ba3MoNbO8.5, Ba3WNbO8.5, and Ba3W1.2Nb0.8O8.6 are very similar (sigma(b) = 0.0022, 0.0017, and 0.0016 S cm -1 , respectively). The variable-temperature neutron diffraction results reported here demonstrate that Ba3W1.2Nb0.8O8.6 undergoes a similar structural rearrangement to Ba(3)MoNbO(8.5 )above 300 degrees C, but the ratio of (W/Nb)O-4 tetrahedra to (W/Nb)O-6 octahedra rises at a faster rate upon heating between 300 and 600 degrees C. There is a clear relationship between the ionic conductivity of Ba3M'M1+x '' O-1-x(8.5+x/2) (M' = W, Mo; M '' = Nb) phases and the number of tetrahedrally coordinated M' and M '' cations present within the crystal structure.