The bilayer to hexagonal phase transition of dioleoylphosphatidylethanolamine has been detected for the f'trst time by differential scanning calorimetry. The observed transition is dfpendem on scan rate. This dependence can be explained by assuming that at rapid scan rates, the rate of conversion of bilayer to hexagonal phase is too slow at low temperatures for equilibration to take place. At higher temperatures the rate of interconversion becomes more rapid. The transition is observed to occur at 14ยฐC using a scan rate of 0.74 K/min while it is centered at 8ยฐC using a scan rate of 0.19 K/min. The enthalpy of the transition is 290 ยฑ 40 cal/mol lipid and the transition is characterized by a ACp of --9 ยฑ 1 mcal K -t (g lipid) -~. The bilayer to hexagonal phase transition of dielaidoylphosphatidylethanolamine and of l-palmitoyl-2oleoylphosphatidylethanolamine occurs at 65.6ยฐC and 71.4ยฐC, respectively, with a corresponding transition enthalpy of 450 ยฑ 20 and 400 ยฑ 30 cal/mol lipid. The transitions of?these phosphatidylethanolamines, occuring at higher temperatures, are independent of scan rate and show a higher degree of cooperativity than that of dioleoylpho~hatidylethanolamine. Compared with the gel to liquid-crystalline transition of bilayer phospholipids the transition to hexagonal phase has a much lower entha~py.