Previous microscopic observations of the polymerizable, diacetylenic lipid 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine in ethanol/water solutions have revealed the formation of thin, undulating filaments upon cooling from 70 degrees C. Infrared spectroscopy and differential scanning calorimetry were used to investigate the phase characteristics of this lipid in mixtures of ethanol and water in order to describe the thermodynamics of filament formation in terms of specific structures. A calorimetric transition was observed at the;concentration of lipid at which filaments form (above 10 mg/mL) at approximately 53 degrees C. Infrared spectroscopy has revealed that the population of various gauche conformer classes, particularly (kink + gtg) and double gauche forms, decreased upon formation of filaments. Filaments are first microscopically observed near 45 degrees C and were stable for at least several hours. The maximum number. density of filaments was observed at between 38 and 40 degrees C at a lipid concentration of 20 mg/mL. Further cooling to 34 degrees C resulted in lipid precipitation and disappearance of the filaments. These results suggest the filaments form from an isotropic to fluid lamellar phase transition as the lipid is cooled in the ethanol/water solution.