The majority of work on the oxygen ion transport properties of electrolyte materials has been undertaken using AC impedance spectroscopy or other electrical techniques. In general, these techniques are excellent methods for measuring the oxygen mobility of materials, such as gadolinia-doped ceria (CGO), due to their negligible electronic conductivity. However, it is also useful to measure the oxygen ion diffusivity of materials directly using instruments such as secondary ion mass spectrometers (SIMS) to verify these conductivity results. The added advantage of a technique such as SIMS is that it also provides data on the kinetics of oxygen exchange on the surface of the material as well as the oxygen diffusion in the bulk. However, there is very little work published to date on the measurement of oxygen ion diffusivity using SIMS on this type of electrolyte material. This paper will concentrate on the oxygen diffusion and oxygen surface exchange behaviour of Ce0.9Gd0.1O1.95 measured by isotope exchange and SIMS. Data will be presented for the dependence of the tracer diffusion coefficient, DT, and the oxygen surface exchange coefficient, k, both as functions of temperature and oxygen partial pressure, P-O2. These results will provide the basis for determining the mechanism for the incorporation of oxygen into these materials.