The oxide ionic conductor Ba3W1.2Nb0.8O8.6 has been synthesised as part of an investigation into the new class of Ba3M’M’’O8.5 (M’ = W, Mo; M’’ = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W6+ for Nb5+ in Ba3W1+xNb1-xO8.5+x/2 leads to an increase in the oxygen content, which enhances the low temperature ionic conductivity. However at 400 °C, the ionic conductivity of Ba3W1.2Nb0.8O8.6 is still significantly lower than the molybdenum compound Ba3MoNbO8.5. Remarkably, at 600 °C the bulk oxide ionic conductivities of Ba3MoNbO8.5, Ba3WNbO8.5 and Ba3W1.2Nb0.8O8.6 are very similar (σb = 0.0016 S cm-1, 0.0017 S cm-1 and 0.0022 S cm-1 respectively). The variable temperature neutron diffraction results reported here demonstrate that Ba3W1.2Nb0.8O8.6 undergoes a similar structural rearrangement to Ba3MoNbO8.5 above 300 °C but the ratio of (W/Nb)O4 tetrahedra to (W/Nb)O6 octahedra rises at a faster rate upon heating between 300 and 600 °C. There is a clear relationship between the ionic conductivity of Ba3M’1+xM’’1-xO8.5+x/2 (M’ = W, Mo; M’’= Nb) phases and the number of tetrahedrally coordinated M' and M" cations present within the crystal structure.