13C NMR was applied to investigate the various phases formed in aqueous mixtures of heptaethylene glycol dodecyl ether (C12E7). On the basis of the analysis of NMR line width, a qualitative picture was drawn concerning the molecular motion of C12E7 in some mesophases as follows. In the H1 and LR phases, the motion of the alkyl chain of the surfactant molecule is rather restricted, whereas the polyoxyethylene (POE) chain can undergo segmental motion to some extent. The molecular motion of the surfactants in the V1 phase, which appears between the H1 and LR phases, is quite rapid and is close to that in the liquid phase. The V1 phase is known to show the highest bulk viscosity in the mesophases due to the network structure constituted by the surfactant bilayer. The rapid motion of C12E7 in the V1 phase suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer. The signal of carbon atoms in the POE chain was shifted upfield by the addition of water in a manner that the extent increased with the increase in water content in the mixture, which reflects the hydration of the POE chain. In addition, the signal of thePOEcarbons was shifted downfield rather continuously by the temperature rise, suggesting that the dehydration of the POE chain proceeds with the increase in temperature.