Ultrasonic attenuation spectra (100 kHz to 2 GHz) and complex dielectric spectra (300 kHz to 40 GHz) of aqueous solutions of 1,2-dimyristoyl-L-3-phosphatidylcholine vesicles are reported and are discussed in view of their behavior near the main phase transition of the lipid. The relaxation terms in the spectra are assigned to the domain structure fluctuations of the membranes, the structural isomerization of alkyl chains, the axial diffusion of lipid molecules within the membrane, and the reorientational motions of the zwitterionic phospholipid headgroups. The relaxation times of the alkyl chain isomerization and of the headgroup motions on the bilayer surfaces show a steplike change at the transition temperature, T-m. The axial diffusion and the domain fluctuations exhibit substantial effects of slowing near T-m as characteristic for critically demixing liquids at their consolute point.