Grain boundaries in ZrO₂ may act as favourable pathways for species, such as oxygen and hydrogen, which play an important role in corrosion when compared with volume diffusion through the bulk of the crystalline material. It is known that segregation of impurity and alloying elements can lead to highly-doped grain boundaries with amorphous structure. Here, these amorphous structures are compared to crystalline materials of equivalent composition. Atomic scale modelling methods have been used to analyse diffusion in undoped systems and zirconia cells doped with 5.3 at. % and 11.0 at. % of trivalent lanthanide species. Diffusion coefficients, pre-exponential factors and activation energies are reported. Oxygen diffusivity was markedly increased in amorphous doped and undoped ZrO₂ systems compared to equivalent undoped crystalline systems. Similar diffusivities are reported between amorphous and crystalline doped systems at the concentrations considered.