The temperature T-m characterizing the gel to liquid crystalline transition in vesicles formed from dialkyl phosphates increases with an increase in alkyl chain length for phosphates with two identical alkyl groups. The temperature T-m also depends on the counter cation. Calorimetric data are interpreted in terms of cooperative melting in domains characterized by patch numbers. The scans recorded by differential scanning microcalorimetry show two or more extrema where the alkyl chain lengths differ.