Oxygen‐18 exchange between gaseous oxygen and cadmium oxide crystals has been studied in the temperature range 630° to 855°C. Since isotope equilibrium is not instantaneously established at the crystal surface an appropriate solution for this particular diffusion problem has been given. In this way it is possible to evaluate rate constants (K) of the phase boundary reaction as well as lattice diffusion constants (D):D=3.8·106exp(−92±4/RT);  K=1.3·102exp(−49±11/RT).The influence of the ambient oxygen pressure (D∼po2−1/5) indicates that a vacancy mechanism is operative in the diffusion of oxygen. This is substantiated by the finding that diffusion is enhanced by incorporation of Li2O. From experiments with doped crystals the fraction of vacancies can be estimated, e.g., 4.4·10−4in ``pure'' CdO at 790°C and 0.16 atm O2. On the basis of thermodynamic considerations the energy of formation of defects can be estimated at 29 leaving 63 kcal/mole for the activation energy of the diffusion process proper.The rate constant of the phase boundary reaction increases with decreasing oxygen pressure (K∼po2−⅓) as well as with increasing Li2O addition. These findings offer a possibility for the suggestion of a mechanism of the surface exchange process.