Self-assembled monolayers, bilayers, and other structures of mixed phosphonic acids were investigated using atomic force microscopy. Octadecylphosphonic acid (OPA, 18 carbon atoms in the alkyl chain) was mixed in solution, with different ratios, with octylphosphonic acid (OcPA, 8 carbon atoms in the alkyl chain) and deposited onto mica substrates. Self-assembled monolayers were formed, with no evidence of phase separation, from pure OPA up to 5: 10 OPA/OcPA ratio (despite the large alkyl chain length difference). It was found that water plays a crucial role in the morphology of the self-assembled structures and their time evolution. The formation of bilayers instead of monolayers (OcPA-type behavior) is dominant for high water content (both in solution and/or the atmosphere). Mechanical and thermal resistance tests were performed on mixed and pure samples and revealed different properties that can be correlated to their composition.