The surface coverage of an adsorbed monolayer of octadecylphosphonic acid on mica was varied via solution concentration and immersion time, thereby controlling the surface free energy. For low surface coverage, a stable wetting layer of deposition solution was formed. For slightly greater coverage, the substrate was initially wetted by a thin liquid layer which quickly ruptured, leaving the sample dry. For large coverage, the liquid layer receded from the sample edge. Atomic force microscopy and video microscopy showed that the intermediate regime corresponds to dewetting via nucleation and growth of holes (0.25-5 mu m in diameter) in the liquid layer while the dewetting at large coverage proceeds from the sample edge. The dependence of the wetting mode upon surface coverage and solution concentration was explained in terms of a competition between two rates : the nucleation rate of holes in the liquid layer and the speed at which the three-phase line advances.