The lattice and dislocation diffusion coefficients for Ca2+in single‐crystal MgO have been determined in the temperature range 1743 to 1293 K from the diffusion penetration determined using secondary ion mass spectrometry. The dislocation diffusion coefficients were 4 to 5 orders of magnitude larger than the lattice diffusion coefficients in the same temperature range. The activation energy of Ca2+lattice diffusion was comparable to that of ionic conductivity. The diffusion mechanism of Ca2+is related to Mg vacancie