Semiconductor nanoparticles, NPs, exhibit fluorescence properties that are closely related to the nature of their surface. CdSe NPs that range in size from 2 to 15 nm in diameter fluoresce with both near-band-edge emission and deep-trap emission, depending on the quality of the surface. When butylamine is added to colloidal solutions of NPs in this size range, the amine is thought to bind to the NP surface and eliminate radiant recombination pathways that lead to luminescence. Since the amine binds to the NP surface, the decrease in fluorescence intensity does not follow standard molecular models of collisional fluorescence quenching. NPs that are smaller than similar to2 nm are composed entirely of a discontinuous arrangement of atoms that are all in constant chemical contact with capping material, solvent, and contaminants. When butylamine is added to solutions of these smaller NPs, the fluorescence quenching follows more standard collisional quenching models. Thus, by monitoring the interaction between NPs and a hole acceptor such as butylamine, one can observe the transition from NP, with atoms both in the core and on the surface, to molecular cluster, with no core atoms to dominate the electron density, but with only surface atoms that can participate in molecular processes such as electron transfer.