Colloidal nanoparticles, comprised of gold nanocrystals, of mean diameter 2.8 nm, coated with an insulating chemically stable self-assembled monolayer of dodecanethiol, have been prepared. Monolayers of nanoparticles have been physisorbed on highly oriented pyrolitic graphite, first by self-assembly, and second by assembly as Langmuir films and subsequent deposition. Nanoparticles have been self-assembled on gold, and immobilized by chemisorption, using decanedithiol during assembly as a linking molecule. Scanning tunneling microscope images of the monolayers are obtained. At high substrate–tip voltages, >0.6 V, the tip is able to climb above the nanoparticles. The tunneling is then a two-step event, tunneling from the substrate to the gold nanocrystal, and subsequently from the gold nanocrystal to the tip. At low voltage, 0.25 V, the Coulomb blockade prevents one extra electron occupying the gold nanocrystal. The tip cannot then climb above the nanoparticles. The theoretical threshold of the blockade is estimated from the nanoparticle size, and shown to be consistent with the observations. At low substrate–tip voltages, rastering of the tip sweeps the nanoparticles from the raster area (but not at high tip voltage). This result has not been described previously, and it is envisaged that it could be used to separate nanoparticles of differing size. However, immobilized isolated nanoparticles are not removed, but only pushed temporarily aside by the scanning tip. © 2001 American Institute of Physics.