In this Letter, we report on the use of atomic force microscopy to study the thermal stability of self-assembled monolayers (SAM) of octadecylphosphonic acid (OPA) grown on mica. The samples were sequentially annealed for 30 min steps at temperatures ranging from 50 to 200 degrees C. A major change in the SAM morphology takes place at temperatures similar to 95 degrees C, with the disappearance of the original flat-topped and partial-coverage SAM morphology. Concomitantly, the nucleation and growth of OPA precipitates in the investigated temperature range are observed. Two separate regimes are identified: initially, for temperatures up to similar to 110 degrees C, nucleation of OPA precipitates occurs, increasing their number density. Then, for temperatures above similar to 10 degrees C, growth of OPA precipitate size dominates, with the density of precipitates remaining approximately constant. A simple model used to fit the experimental data of both regimes enables an estimate of similar to 9 kcal/mol as the energy required for the disorganization (Ed) of OPA molecules on mica.