Dense CeO2 layers of 350 nm thickness, with columnar grains of size 25 nm, were RF-sputtered on silicon. O-18 isotope exchange in the temperature range from 200 to 575 degrees C showed surface exchange at CeO2 as the rate-determining step, while diffusion through the polycrystalline thin layer was fast. The surface exchange coefficients controlled by surface or grain boundary processes were found to be k(s) = 2.7 x 10-8 exp(-0.3eV/kT)cm s(-1) or k(gb) = 1 x 10(-9) exp(-0.3eV/kT)cm s(-1.) Comparison with literature data on doped ceria revealed that surface exchange could be dominated by grain boundary processes. A lower limit for the diffusion coefficient was determined as 10(-15) cm(2) s(-1) at 575 degrees C. (c) 2007 Elsevier B.V. All rights reserved.