We have observed the formation process of octadecylphosphonic acid self-assembled monolayers on mica in situ, at the liquid/solid interface, using atomic force microscopy. The submonolayer island nucleation rate was found to depend on the concentration of the deposition solution, which affects the deposition rate of adsorbate molecules on the substrate. The power-law dependence of the submonolayer island density on the solution concentration was consistent with the interpretation that the minimum size of a stable island is two molecules. Four distinct regimes of growth were found: a low-coverage nucleation regime, an intermediate coverage regime, an aggregation regime in which the island density remained constant, and a coalescence regime. The island density kinetics in the first two regimes were compared with the predictions of a kinetic theory of 2D cluster growth typically used to describe vapor phase molecular beam epitaxy at low temperatures.