Charge carrier trapping is an important phenomenon in nanocrystal (NC) decay dynamics because it reduces photoluminescence (PL) quantum efficiencies and obscures efforts to understand the interaction of NC excitons with their surroundings. Particularly crucial to our understanding of excitation dynamics in, e.g., multiNC assemblies, would be a way of differentiating between processes involving trap states and those that do not. Direct optical measurement of NC trap state processes is not usually possible because they have negligible transition dipole moments; however, they are known to indirectly affect exciton photoluminescence. Here, we develop a framework, based on Marcus electron transfer theory, to determine NC trap state dynamics from time-resolved NC exciton PL measurements. Our results demonstrate the sensitivity of PL to interfacial dynamics, indicating that the technique can be used as an indirect but effective probe of trap distribution changes. We anticipate that this study represents a step toward understanding how excitons in nanocrystals interact with their surroundings: a quality that must be optimized for their efficient application in photovoltaics, photodetectors, or chemical sensors.quantum dot ͉ states ͉ electron transfer ͉ time-correlated single-photon counting ͉ fluorescence intermittency C olloidal semiconductor nanocrystals (NCs) are potentially useful in a variety of photoactive applications because of their widely tunable electronic band gaps and their ease of processability. Different from well-studied molecular fluorophores, the excited electronic states of NCs with diameters in the range 1 to 10 nm are examples of nanoscale excitons (1). To be used in solar photovoltaics, for example, photo-generated NC excitons must be able to dissociate and transfer charge carriers to their surroundings in a controlled fashion; however, this process is impeded in NCs by the considerable influence of surface-localized states that trap carriers (2). Perturbations due to traps are important because of the significant surface-to-volume ratios characteristic of small colloids (3, 4). Certain surface sites act as charge acceptors that dissociate excitons and therefore reduce the photoluminescence (PL) quantum yield. Changes in solvent or surface-bound coordinating ligands have been found to affect these surface traps and thereby influence steady-state (5-9) and time-resolved (10-12) NC PL. The ability of surface traps to act as charge acceptors makes them excellent model systems for elucidating exciton dissociation processes occurring on the NC surface. Properties intrinsic to NC excitons have been examined in detail (13-17) but comparatively little is understood about the interplay among intrinsic excitons and surface states. Here, we report investigations of CdSe/CdS/ZnS core/shell/shell NC ensemble PL and establish a quantitative model of interfacial trapping and its effect on NC PL dynamics.Colloidal CdSe NCs are coated with passivating ligands that sustain their dispersion in solution and minimize ...