Complex electric pemittivity spectra of aqueous vesicle solutions, prepared by extrusion of lecithin suspensions, have been measured as a function: of-temperature in order to study the dielectric relaxation of the dipolar phospholipid headgroups. Solutions of unilamellar Liposomes from dimyristoylphosphatidylcholine and from a mixture of dimyristoyl- and dipalmitoylphosphatidylcholine (mole fraction x = 0.5) have been investigated. Based on a theoretical model that considers the internal electric fields of the dielectrically heterogeneous liquids, the vesicle radius, the zwitterion relaxation time, a relaxation time distribution parameter, and a quantity that is directly related to the dipole orientation correlation factor are calculated from the measured spectra. The radii indicate a noticeable swelling when T exceeds the gel/fluid phase transition temperatures T-m of the membranes. There clearly exists an effect of correlation of dipole orientations. Either the dipole orientation correlation factor or the radius of the circular path of headgroup motions around the phosphate group is substantially higher at T > T-m than at T < T-m. The head group mobility in bilayers is smaller than in micellar systems made of lysolecithins, but it also increases noticeably when T exceeds T-m. With lecithin mixtures, in addition to the fluctuations in the domain structure, the clustering of like and unlike lipid molecules obviously also contributes to the broadness of the relaxation time distribution.