Monolayers of poly(ethylene glycol) 1500 were grown by dispersing and drying a dilute solution of the polymer on a silica substrate. Atomic force imaging of these thin films reveals self-assembly of vertically oriented polymer chains that eventually leads to dendritic structures on a mesoscopic length scale. Thermal annealing and desorption of such structures were investigated under ultrahigh vacuum using X-ray photoelectron spectroscopy and temperature-programmed desorption. The carbon Is signal of the polymer starts to decrease before the onset of desorption. This observation can be rationalized by structural reordering of the polymer chains on the surface. Atomic force microscopy images prove that the decrease of the characteristic X-ray photoelectron spectroscopy signal at low temperatures is correlated with the transition of the two-dimensional dendritic structures to three-dimensional microdroplets of the polymer on the surface. As the temperature increases, this surface dewetting process is followed by dissociative desorption of the polymer, shrinking of the microdroplets, and finally complete depletion of the surface.