Composites consisting of a perovskite-based electronic or mixed conductor with a fluorite-structured ionic conductor are often used as electrodes in solid oxide electrochemical energy conversion devices. After sintering the materials, there is often evidence for inter-reaction between the two phases, or inter-diffusion of cations or impurities between the two phases. We studied the 18O exchange properties of a composite consisting of CGO and LSCF in a 50 : 50 ratio. High resolution ToF-SIMS mapping reveals that the 18O fraction at the very outer surface of grains of the CGO phase is much higher than expected from D* and k* values for the single-phase parent material. Surface compositional analysis by ToF-SIMS and low energy ion scattering (LEIS) spectroscopy suggests that the surfaces of the CGO grains in the composite do not show the impurities which typically segregate to the surface in single-phase CGO. Thus, the “cleaning” of impurities from the CGO surface by dissolution into the perovskite phase may be one explanation for the apparent enhanced surface exchange for CGO in these composites.