Self-assembled monolayers are formed by exposing freshly cleaved mica to a solution of octadecylphosphonic acid in tetrahydrofuran. Atomic force microscope images of samples immersed in solution for varying exposure times show that prior to forming a complete monolayer the molecules aggregate into dense islands (1.8 ± 0.2 nm high) on the surface. The islands have a compact, rounded morphology. The cosine of the contact angle between hexadecane and the partial monolayers has an approximately linear dependence on coverage. However, the cosine of the contact angle of water decreases linearly to about 50% coverage (the percolation threshold of the phosphonate islands) and then appears to saturate.