Monolayer-protected nanoparticles (MPNs) with average core sizes of 1.7- (small): 2.2- (medium) and 4.5-nm (large) diameter have been prepared and functionalized with a variety, of aryl ketone substrates, namely, 11-mercaptoundecaphenone (1), 1-(4-hexyl-phenyl)-11-mereaptoundeeanone (2). 1-[4-(11-mercaptoundecyl)phenyl]hexanone (3), or 1-[4-(11-mercaptoundecyl)phenyl]undecanone(4). Upon irradiation in benzene solution, the aryl ketone-modified MPNs undergo the Norrish type II photoreaction and yield alkene- or acetophenone-modified MPNs exclusively, with no evidence for the generation of cyclobutanol. The extent of the photoreaction for the entire series of aryl ketones is dependent on the size of the MPN core. For 11-mercaptoundecaphenone, the reaction proceeds nearly to completion on the smallest MPN cores (99 +/- 1%) but occurs to a much lesser extent on medium (85 +/- 5%) and large cores (66 +/- 6%). The differences in the extents of reaction are rationalized by the decreased reactivity of substrates on terrace regions, which become increasingly larger with the core size. In lending support to this hypothesis, the edge and vertex sites of medium-sized MPNs were selectively populated with an aryl ketone probe and shown to react quantitatively, whereas selective population of the terrace sites on the same-sized MPNs results in a much lower extent of reaction. Together, these results indicate differences in reactivity of monolayer substrates on terrace versus edge/vertex sites of MPNs. The differences in reactivity with site will play a role in the design of modified MPNs for applications.