Noble metal nanoparticles made of gold, silver, and copper are used as catalysts in a wide range of processes, from pollution mitigation, to the synthesis of liquid fuels and chemical precursors. Recently, it has been shown that irradiating such nanoparticles with light can significantly improve their catalytic activity and selectivity, a result that could have a tremendous impact on the chemical industry. The observed optical modulation of the electronic properties of nanostructured catalysts is due to the excitation of plasmon resonances, which are collective oscillations of the nanoparticle’s free electrons. However, despite increasing experimental efforts, the exact physical mechanism behind the observed plasmon enhancement of catalysis is still a topic of intense debate. Three parallel processes are being proposed as potential explanations: a) plasmon resonance decay into non-equilibrium “hot” electrons, b) plasmon thermalization and consequent “heating” of the nanoparticles, and c) strong light absorption and localization in electromagnetic “hot” spots at the surface of the nanoparticles. In this proposal I will quantitatively discriminate between these competing “hot” mechanisms of optical activation of catalysis, by comparing the electron transfer rates in model catalytic reactions in the dark and under plasmon resonance excitation. In particular, I will measure the reaction conversion rate on nanocatalysts with well-defined size, shape, composition, and surface chemistry, while selectively turning off each “hot” activation mechanism, by rational choice of the optical and chemical parameters. Furthermore, I will use a super-resolution microscopy approach to spatially reconstruct the distribution of catalytic events at the surface of individual nanoparticles, which will allow a direct visual discrimination between the competing mechanisms of plasmon activation of catalysis. Together, these results will provide the first quantitative assessment of the potential of plasmonics in catalysis and will open new pathways to control and activate chemical reactions using light.