Ex-situ and in-situ studies were performed for self-assembled monolayers (SAMs) formation on mica. The behavior of two surfactants [octadecylphosphonic acid (OPA) and dodecylphosphonic acid (DPA)] in two different solutions [ethanol or tetrahydrofuran (THF)] were studied at different concentrations. A cross section analysis of the monolayer images at partial coverage showed thickness of 1.8 nm, 0.7 nm, and 1.0 nm for OPA, DPA and mixed OPA/DPA, respectively. These values are similar to the ones presented by Xiao and those expected for a monolayer of mixed surfactants as proposed by Whitesides. Since the topography of a monolayer resembles the substrate, the presence of the monolayer at full coverage was determinated by force-distance curve measurements. It was observed that the solvent had the ability to re-dissolve the OPA monolayer, suggesting that the bonding is of a physisorption nature.