Differential Scanning Calorimetry (DSC) was used to study the effect of the incorporation of a series of semifluorinated diblocks FnHm (F6H10, F6H16, F8H14, F8H16, F8H18 and F8H20) on the gel and liquid states of the bilayer of large multilamellar DMPC and DPPC liposomes. The presence of the FnHm diblocks affects slightly the T-m of the main gel-liquid transitions of DMPC and DPPC, but is accompanied by the appearance of a second transition in the calorimetric traces whose T-m is mainly determined by the length of the F-n segment. The DSC results are consistent with the previously established conclusion that the F-n segments of the diblocks form a central layer in the core of the lipid bilayer, with the H-m segments being interdigitated with the lipid chains. The DSC traces suggest that the structure of the fluorinated liposomes is a double bilayer at 3:4 and 1:2 and a trilayer at 2:1 lipid/FnHm molar ratios. At temperatures between the two phase transitions T-m's, the fluorinated liposomes are neither in a gel-like or a liquid-like state but rather posses both characteristics. (C) 2010 Elsevier Inc. All rights reserved.