Nonlinear optical Sum and Difference-Frequency spectroscopies are used to probe and model the surface of thiophenol-functionalised gold nanoparticles grafted on a Si(100) substrate through two different silanization procedures. By scanning the [980-1100 cm À1 ] infrared spectral range with the CLIO Free Electron Laser, ring deformation vibrations of adsorbed thiophenol are investigated. Quantitative data analysis addresses three levels of organization: microscopic, nanoscopic and molecular. Grafting with p-aminophenyl-trimethoxysilane shows an increase of around 40% in surface density of nanoparticles (N s) as compared to 3-aminopropyl-triethoxysilane. The relative amplitudes of the resonant and nonresonant contributions to the SFG and DFG spectra are discussed in terms of N s , Fresnel reflectivity factors and local amplification of the nonlinear signals by coupling to the surface plasmon of the particles. They are shown to quantitatively scale with N s , as measured by atomic force microscopy. Vibration mode assignment is performed through a critical analysis of literature data on IR and Raman spectroscopies coupled to DFT calculations, for which a methodology specific to molecules adsorbed on gold atoms is discussed.