We report an experimental study of Langmuir monolayers of dodecanethiol-ligated gold nanoparticles with in situ optical microscopy and X-ray scattering. The particle spacing increases with thiol concentration but does not depend on surface pressure. Compression beyond the full coverage of the monolayers causes the monolayers to wrinkle. With the presence of excess thiol molecules, the wrinkles unfold back to monolayer upon the expansion of the surface area. A theoretical model based on van der Waals' attraction and tunable steric repulsion is adopted to explain this reversibility. (c) 2006 Elsevier B.V. All rights reserved.