The kinetics of the NO + CO reaction from 433 to 573 K were studied over supported Pd with and without ceria promotion. In addition, temperature-programmed desorption (TPD) of products formed from preadsorbed NO was performed. For the ceria-promoted samples, the interface between Pd and ceria was changed by varying particle size and the amount of ceria postdeposited on the Pd particles. The Pd particle size affected both activity and apparent reaction orders. For the unpromoted catalyst, the Pd particle size of about 4.5-nm average diameter, determined by dihydrogen chemisorption, had a lower turnover frequency than 1-mn size particles, indicating structure sensitivity of the NO + CO reaction. However, the ceria-promoted samples showed a maximum in catalytic activity for 2-nm Pd. This maximum in reaction rate suggests a balance between the Pd/ceria interface which promotes NO dissociation and the Pd metal surface which catalyzes product formation. Results from TPD of adsorbed NO are consistent with this interpretation. High-resolution transmission electron microscopy was used to confirm the proximity of small ceria crystallites to the supported Pd particles.