Lead sulfide, PbS, nanoparticles with the average size of 3.2 +/- 0.4 nm have been synthesized by exposing self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid adsorbed Pb2+ ions on Au(111) substrate into a H2S atmosphere. The preparation process of the PbS nanoparticles consists of the formation of the SAMs, deprotonation, and Pb2+ ion adsorption of outer surface of the SAMs, followed by a gas/solid chemical reaction. The whole process was traced by infrared reflectance spectra with grazing incidence reflection mode and X-ray photoelectron spectroscopy. In addition, scanning tunneling microscopy and high-resolution transmission electron microscopy images provided the direct proof of formation of the small-sized PbS nanoparticles. A possible nucleation model of PbS growth on the SAM has been suggested.