Abstract Bulk and grain boundary diffusion of Nb5+ cations in yttria-stabilized zirconia (YSZ, 8 mol% Y2O3–92 mol% ZrO2) and in titania-doped yttria-stabilized zirconia (Ti–YSZ, 5 mol% TiO2–8 mol% Y2O3–87 mol% ZrO2) was studied in air in the temperature range from 900 to 1300 °C. Experiments were performed in the B-type kinetic region. Diffusion profiles were determined using the secondary ion mass spectrometry (SIMS). The temperature dependencies of the bulk diffusion coefficient D and the grain boundary diffusion parameter D′δs for both the materials were calculated. The activation energies of these transport processes in YSZ amounts to 258 and 226 kJ mol−1, respectively, and 232 and 114 kJ mol−1 in Ti–YSZ. The results were compared to the diffusion data of other cations previously obtained for the same material.