Monomolecular films of phospholipids in the liquid-expanded (LE) phase after supercompression to high surface pressures (), well above the equilibrium surface pressure (_e) at which fluid films collapse from the interface to form a three-dimensional bulk phase, and in the tilted-condensed (TC) phase both replicate the resistance to collapse that characteristizes alveolar films in the lungs. To provide the basis for determining which film is present in the alveolus, we measured the melting characteristics of monolayers containing TC dipalmitoyl phosphatidylcholine (DPPC) as well as supercompressed 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) and calf lung surfactant extract (CLSE). Films generated by appropriate manipulations on a captive bubble were heated from 27°C to 60°C at different constant above _e. DPPC showed the abrupt expansion expected for the TC-LE phase transition, followed by the contraction produced by collapse. Supercompressed CLSE showed no evidence of the TC-LE expansion, arguing that supercompression did not simply convert the mixed lipid film to TC DPPC. For both DPPC and CLSE, the melting point, taken as the temperature at which collapse began, increased at higher , in contrast to POPC, for which higher produced collapse at lower temperatures. For between 50-65 mN/m, DPPC melted at 48-55°C, well above the main transition for bilayers at 41°C. At each , CLSE melted at temperatures > 10°C lower. The distinct melting points for TC DPPC and supercompressed CLSE provide the basis by which the nature of the alveolar film might be determined from the temperature-dependence of pulmonary mechanics.