The assembly of nanoparticles into large, two-dimensional structures provides a route for the exploration ofcollective phenomena among mesoscopic building blocks. We characterize the structure of Langmuirmonolayers of dodecanethiol-ligated gold nanoparticles with in situ optical microscopy and X-ray scattering.The interparticle spacing increases with thiol concentration and does not depend on surface pressure. Thecorrelation lengths of the Langmuir monolayer crystalline domains are on the order of five to six particlediameters. Further compression of the monolayers causes wrinkling; however, we find that wrinkled monolayerswith excess thiol can relax to an unwrinkled state following a reduction of surface pressure. A theoreticalmodel based on van der Waals attraction and tunable steric repulsion is adopted to explain this reversibility.