Heterogeneous metal nanocatalysts have recently emerged as attractive catalysts for a variety of couplings (e.g., C–C, C–N, C–S, C–O, etc.). However, the characterization of the catalytic pathway remains challenging. By exploiting localized surface plasmon resonance (LSPR) of the catalytically relevant gold (Au) nanostructure, we show that UV–vis spectroscopy can be used to confirm the homogeneous catalytic pathway. Specifically, we have demonstrated that Au nanoparticles under C–C coupling conditions undergo substrate-induced leaching to form homogeneous Au catalytic species. The LSPR spectroscopic approach opens a new door to track stability of nanocatalysts and characterize the catalytic pathway in a range of coupling reactions.