Isotopic Exchange Depth Profiling (IEDP) using Secondary Ion Mass Spectrometry (SIMS) has been used to investigate oxygen self-diffusion and surface exchange reactions in the related fluorites, yttria stabilised zirconia (YSZ) and gadolinia doped ceria (CGO). The nature of the exchange reaction between the gas and the surface of the solid has been explored using the gas phase analysis technique. Both YSZ and CGO samples showed a change in the activation energy of the surface exchange process at approximately 700 degrees C. In both temperature regimes the two materials show similar activation enthalpies for the exchange reaction. At temperatures below 700 degrees C the activation energy for surface exchange is similar to that for bulk oxygen diffusion within the same sample (i.e. approximately 1 eV for YSZ and 0.9 eV for CGO). At temperatures greater than 700 degrees C a marked increase in the activation energy for surface exchange was observed, whilst no discernible change was seen for bulk oxygen transport. The activation energy for surface exchange of YSZ was observed to be 2.3 eV at temperatures greater than 700 degrees C, whilst for CGO the activation energy increased to 3.3 eV.