Dark-field scattering spectroelectrochemistry is used to analyze the electrochemical formation of individual Ag nanoparticles (NPs) at the surface of an indium tin oxide electrode. Heterogeneities in redox potentials among NPs not visible in bulk electrochemical measurements are presented for the first time. Through correlated electron microscopy, single NP light scattering intensity is related to particle size according to Mie theory, enabling rapid particle size determination and the construction of voltammetric curves for individual NPs.