Oxygen diffusivity in oxides is usually determined via gas/solid exchange experiments in an atmosphere enriched as to the concentration of the rare stable isotope O-18. The kinetics of the O-16 and O-18 isotope exchange at the surface is usually described by the surface exchange coefficient, K, The analytical solution of the diffusion problem suggests the definition of a characteristic time constant, tau equivalent to D/K-2. This time constant determines the duration that is necessary to reach equilibrium between the O-18 gas concentration and the O-18 concentration at the surface of the solid. The simultaneous determination of D and K by secondary ion mass spectrometry (SIMS) depth profiling works well in a large parameter range of D and K. However, at low temperatures, the simultaneous determination of D and K may be subject to severe errors. To overcome this problem, the possibility to determine D and K separately will be discussed.