The dissociative binding efficiency of oxygen over Pd(n)/TiO(2)(110) (n = 4, 7, 10, 20) has been measured using temperature programmed reaction (TPR) mass spectrometry and X-ray photoemission spectroscopy (XPS) following exposure to O(2) with varying doses and dose temperatures. Experiments were carried out following two different O(2) exposures at 400 K (10 L and 50 L) and for 10 L of O(2) exposure at varying temperatures (T(surf) = 200, 300, and 400 K). During TPR taken after sequential O(2) and CO (5 L at 180 K) exposures, unreacted CO is found to desorb in three features at T(desorb) ≈ 150, 200, and 430 K, while CO(2) is observed to desorb between 170 and 450 K. We show that Pd(20) has exceptionally high efficiency for oxygen activation, compared to other cluster sizes. As a consequence, its activity becomes limited by competitive CO binding at low O(2) exposures, while other Pd(n) sizes are still limited by inefficient O(2) activation. This difference in mechanism can ultimately be related back to differences in electronic properties, thus making this question one that is interesting from the theoretical perspective. We also demonstrate a correlation between one of the two CO binding sites and CO(2) production, suggesting that only CO in that site is reactive.