The impurity diffusion of Hf and Zr in Gd-doped CeO2 was examined by means of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Dense, polycrystalline samples with a dopant site fraction of n(Gd) = 0.5% were subjected to diffusion anneals in the temperature range 1400 <= T/K <= 1600 in ambient air. Isothermal values of the bulk diffusion coefficient, D-b, were similar for the two isovalent impurity cations. The activation enthalpies of bulk diffusion were also similar, Delta H-Db(Zr) = (5.5 +/- 0.4) eV and Delta H-Db(Hf) = (5.3 +/- 0.3) eV. Fast grain-boundary diffusion of Hf was observed (the high background of Zr prevented the analysis of Zr grain-boundary diffusion). The activation enthalpy of the grain-boundary diffusion product, Delta H-omega Dgb(Hf) = (5.9 +/- 0.8) eV, is, surprisingly, similar to the activation enthalpies for bulk diffusion. Attention is drawn to possible problems that may arise when using chemical tracers instead of true (stable or radioactive) tracers. (C) 2017 Elsevier B.V. All rights reserved.