Abstract In a previous publication, (Singer, M.A., Jain, M.K., Sable, H.Z., Pownall, H.H., Mantulin, W.W., Lister, M.D. and Hancock, A.J. (1983) Biochim. Biophys. Acta, 731, 373–377), we reported the properties of aqueous dispersions of the six diastereo-isomers of cyclopentanoid analogues of dipalmitoylphosphatidylcholine. Two of these isomers displayed unusually high enthalpies of transition, about double that of dipalmitoylphosphatidylcholine. One of the high enthalpy isomers whose configuration is all- trans has now been modified by the insertion of extra methylene residues ( n = 3 through 9) between the nitrogen and phosphorus atoms of the headgroup. Vesicles were formed from these lipids and studied by 22Na permeability measurements, differential scanning calorimetry, fluorescence polarization, 31P-NMR, and freeze-fracture electron microscopy. Vesicles composed of lipids with n = 2 or 3 exhibit a sharp transition at 46°C or 49°C, respectively, and a high enthalpy with no detectable sub- or pretransitions. Lipids with n > 3 exhibit a main transition between 38 and 43°C with enthalpies < 10 kcal/mol and after prolonged cooling (more than 3 days at 4°C) a broad endotherm at about 20 ± 3° C with enthalpies > 4 kcal/mol . These same dispersions display a permeability peak at 20–25°C and a second increase in 22Na efflux in the temperature range 30–40°C. The results of 31P-NMR measurements suggest that the acyl chains in 2,3-dipalmitoylcyclopentanol-phosphocholine ( n = 2) bilayers have restricted rotation below the main phase transition temperature.