Ba3MNbO8.5 (M: W and Mo) (Yasui et al. in The Journal of Physical Chemistry C 126:2383–2393, 2022) has received considerable scholarly attention in recent years because of high oxide-ion conductivity and potential use for many applications. This study shows the DC conductivities of Ba3WNbO8.5 (3.7 × 10−2 S cm−1) and Ba3MoNbO8.5 (8.8 × 10−2 S cm−1) at 900 ºC and confirms higher activation energy for conductivity of Ba3WNbO8.5 than that of Ba3MoNbO8.5. Key factors affecting the conductivity and activation energy are the ratio of tetrahedral O3 to octahedral O2 oxide ions and diffusion pathways in Ba3MNbO8.5. However, the O2/O3 occupational disorders and oxide-ion diffusion paths are unresolved important issues in Ba3MNbO8.5. Here, crystal structure and neutron scattering length density (NSLD) analyses of in situ neutron-diffraction data up to 800 ºC were performed on Ba3WNbO8.5 by Rietveld method and maximum-entropy method (MEM). MEM NSLDs show two-dimensional oxide-ion diffusion through the octahedral O2 and tetrahedral O3 sites in the oxygen-deficient c′ layer. Numbers of the O3 and O2 atoms n(O3) and n(O2) increase and decrease, respectively, with an increase of temperature, which indicates that the O2/O3 disorder is more significant at high temperatures. The O2/O3 disordering makes the minimum NSLD on the O2–O3 path higher, oxide-ion conductivity increase, and it leads to higher activation energies of Ba3WNbO8.5 in comparison with Ba3MoNbO8.5.