The dissociative binding efficiency of oxygen over Pd-n/TiO2(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 approximate to 150, 200, and 430 K, while CO2 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 CO2 production, suggesting that only CO in that site is reactive.