Self-assembled monolayer growth of octadecylphosphonic acid on single-crystal C-face (1000) and R-face (11 h02) sapphire (corundum) has been investigated by ex situ tapping-mode atomic force microscopy, contact angle measurements, and Fourier transform infrared spectroscopy. The process of film formation is highly dependent on the temperature, a phenomenon that is explained by a three-phase model for film deposition. At low temperature, close-packed molecular islands nucleate and grow, surrounded by virtually bare substrate, analogous to a two-dimensional (2D) vapor-to-solid transition. At high temperature, on the other hand, the growth involves a 2D liquid-to-solid transition. Different growth kinetics are observed for the two crystal orientations, which can be related to the surface energetics.