Surface-enhanced IR–visible sum frequency generation (SFG) was studied using Ag and Au films over nanospheres (AgFON and AuFON), which provided large area of reproducible nanoscale structures with well-defined morphology. SFG vibrational spectra for a self-assembled monolayer of octadecanethiol were investigated on spheres with diameters ranging from 300 nm to 620 nm. With an input wavelength of 532 nm, a sphere diameter near 360 nm produced the maximum enhancement, which was consistent with the localized surface-plasmon resonant wavelength identified by minimum reflectivity. The measured second-order susceptibility for the asymmetric CH₃ stretch mode of octadecanethiol was enhanced by up to 27 times on AgFON, which corresponded to an SFG enhancement factor of ∼730. The SFG enhancement factor for AuFON was a quarter of that from AgFON. The SFG enhancement factor for the symmetric CH₃ mode was about 10 times smaller than that for the asymmetric CH₃ mode. This difference can be explained by the highly directional electrical-field parallel to the surface, created at the junction between nanospheres. Polarization-dependent studies also indicated that excitations with the electrical field parallel to the surface was the main contribution to the observed surface enhancements, and the electrical field perpendicular to the surface offered little enhancement.