We have investigated how ozone affects high-temperature electrical properties of nanostructured yttria-doped ceria (YDC) thin films using impedance spectroscopy. It is found that predominantly ionic conductivity in the doped-ceria lattice is not altered upon exposure to ozone. The grain boundaries in YDC ultra-thin films, in contrast, exhibited a substantial increase in resistance in ozone-rich environment. While activation energy for lattice conduction in ozone was similar to air, being close to 1 eV, grain boundary activation energy increased notably from ~1.3 eV in air to 1.7 eV in ozone. Also, we have investigated kinetics of resistance change in such thin films in air and in ozone ambient using galvanostatic measurements and found that the rapid resistance change observed in ozone was reversible. Mechanisms leading to the observation and implications for ozone-ceria film surface interactions are discussed.