We report on measurements of the chemical diffusion coefficients of rare earth cuprate single crystals REBa2Cu3O7−δ (RE=Y, Er, Dy) grown in crucibles made of Y2O3-stabilized ZrO2 and BaZrO3. The diffusion coefficients have been determined by in situ measurement of the electrical conductivity using a four point method during the oxygenation at various oxygen partial pressures and in the temperature range of 390°C–550°C. In this range the diffusion coefficient was determined between 3.7×10−9 cm2/s and 6.6×10−8 cm2/s independent of the rare earth atom or of the different impurity levels. For all crystals we show that the in- and out-diffusion process can be described with the same time constant at each annealing temperature. Within the accuracy of ±5% we measure for both processes the same value for the diffusion coefficient. The temperature dependence of the diffusion coefficient obeys an Arrhenius-law for all crystals, proving that the oxygen diffusion in the rare earth RE 123 superconductors is a purely thermally activated process. The values for the activation energy show a scattering up to 30% and vary from 0.76 to 1.09 eV. There is neither a dependence of the activation energy on the central rare earth atom nor on different impurity levels.