Mixed stearic acid-barium stearate Langmuir-Blodgett (LB) monolayers have been deposited on oxygen plasma oxidized silicon substrate. Atomic force microscope (AFM) has been used in order to obtain quantitative information about the coverage of the substrate after annealing at temperatures ranging from room temperature to 120 degrees C. These measurements allowed us to measure the rate constant K of the desorption process. Data showed that at low temperatures the formation of new holes (nucleation) and hole broadening (growth) have comparable rates, while at higher temperature nucleation has a catastrophic effect on the coverage. These observations suggest that different mechanisms are in action at temperatures above and below 55 degrees C, the hydration layer present between substrate and organic film probably playing a role in bath mechanisms. Data are discussed in terms of both activated state theory and dynamical model.