Transport-reaction processes at individualAg nanoparticles (NPs)are studied using electrochemistry coupled with in situ 3D light scatteringmicroscopy. Electrochemistry is used to trigger a (i) diffusiophoretictransport mode capable of accelerating and preconcentrating NPs towardan electrode and (ii) subsequent diffusion-controlled oxidation ofNPs. Individual NP dissolution rate, analyzed using optical modeling,suggests the intervention of insoluble products. New insights intodiverse NPs behaviors highlight the strength of coupled optical-electrochemical3D microscopies for single-NP studies. [ABSTRACT FROM AUTHOR]