Time-resolved atomic force microscopy, contact angle analysis, and X-ray photoelectron spectroscopy were employed to study the effect of cyclic heating and cooling of a partial octadecyltrichlorosilane (OTS) monolayer on the oxidized Si(100) surface. A reversible structural change of the partial monolayer between high density and low density two-dimensional liquid phases was observed as a function of temperature at constant coverage. These studies show that the monolayer exists in a highly mobile hydrogen-bonded state akin to the equilibrium state of Langmuir films at the air-water interface. The lifetime of the mobile state was measured to be on the order of several minutes, after which grafting and cross-linking immobilize the monolayer. The results show that the mobile state permits large scale rearrangements of the molecules within the monolayers.