Abstract: Observation of melting in solid-supported organic monolayers of long-chain amphiphiles using FTIR spectroscopy and wettability measurements is reported. Three different substrate-monolayer interactions-covalent, ionic, and physical bonds-were investigated. Both Langmuir-Blodgett and self-assembly techniques were used. With the exception of covalently bound octadecyltrichlorosilane, all species underwent a large, irreversible randomization at around 110°C. Although heating affected slight disorientation of the chains in QTS, no sharp phase transition characteristic of a melting process could be detected for temperatures up to 140°C. The importance of head group immobilization in the thermal stabilization of monolayer structures is demonstrated.