The temperature dependence of the circular dichroic properties of liposomes from different phosphatidylcholines has been investigated with the aim of clarifying whether circular dichroism (CD) measurements can give information on the conformational flexibility in the region of the head group of phospholipids within the bilayer of liposomes. These CD measurements reflect electronic transitions of the intrinsic ester chromophors localized near the chiral center of the phospholipids, and they support earlier NMR and molecular dynamics studies which have shown that a considerable conformational rigidification in the glycerol part of the phosphatidylcholine molecules in Liposomes exists. These conformational restrictions are more significant at lower temperature, showing a significant increase in CD intensity around 220 nm on cooling the liposomes below the main phase transition temperature T-c. To probe the hydrophobic domain of the liposomes. in which no intrinsic chromophor is localized, the achiral, linear, water insoluble 1,6-diphenyl-1,3,5-hexatriene (DPH) molecule has been solubilized in liposomes. Below T-c DPH shows an (induced) CD spectrum with multiple maximums in the region of the absorption maximums of DPH (300-400 nm), the sign of the CD signal being dependent on the chirality of the phospholipids used. Above T-c, DPH in the liposomes is not CD active. On the basis of these findings, the use of the induced CD of DPH is proposed as an alternative method for the estimation of T-c of phospholipid bilayers.