B-site cation inter-diffusion in the ABO3 perovskite yttrium-substituted barium zirconate (BZY) was studied at temperatures from 1100 to 1460 °C under reducing and oxidising conditions. The experiments followed two different approaches using Ce as a chemical tracer for Zr. By fitting diffusion profiles of the cation obtained by either Electron Probe Microanalysis (EPMA) or Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), we determined bulk, grain-boundary and effective diffusion coefficients. The activation energies of bulk diffusion (4.5 ± 0.4 eV) are similar to the migration enthalpy obtained by a computational approach (∼4 eV). On this basis, we conclude that the activation energies correspond to the migration enthalpies, assuming that the concentration of Zr vacancies in the bulk was frozen-in. Grain-boundary diffusion coefficients were found to be more than four orders of magnitude higher than the corresponding bulk values, while the activation energies of grain-boundary diffusion are relatively similar to those for bulk. This was attributed to higher concentrations of cation vacancies in space-charge layers at the boundaries. Overall, the results show that BZY electrolytes are highly stable towards degradation related to B-site cation diffusion.