The spectrophotometric determination of molar absorptivities of the surface plasmon (SP) resonance for gold and gold–silver nanoparticles is reported. The importance of the optical parameter for assessing concentrations of nanoparticles in solution reactions and for estimating thickness of thin film assembly of nanoparticles is highlighted. The high monodispersity (5.6±0.4 nm) of decanethiolate-capped gold nanoparticles produced by thermally activated processing protocol served as a model system, which yielded an ε value of 1.1×10 7 M −1 cm −1. This value was also compared with values determined similarly for nanoparticles of different core sizes and compositions, including decanethiolate-capped gold (1.9 nm), gold–silver alloy (2.7 and 5.6 nm), and citrate-capped gold nanoparticles (13.4 nm). The determined ε values are used for the thickness estimation of thin film assembly of gold nanoparticles linked by 1,9-nonanedithiol (NDT) and the concentration calculation of nanoparticles in a solution reaction. The results have important implications for quantitative assessment of nanoparticle-related reactivities and structures.