The phase behavior of an aqueous mixture of octaethylene glycol dodecyl ether (C12E8) was investigated by means of differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy, and C-13 nuclear magnetic resonance (C-13 NMR) spectroscopy. The phase diagram in the temperature range from -10 to 70 degreesC was constructed on the basis of DSC experiments, according to which four mesophases, that is, L-a, V-1, H-1, and I-1, appear in this mixture system depending on the composition and temperature. The FT-IR measurements as a function of composition and temperature revealed the events occurring in the mixture associated with the phase transformation. When the mixture transforms from solid to mesophases, the hydrogen bonds between terminal OH groups of the surfactant molecules are broken. Accompanied by this phase transformation, the ordered conformational structure of both the polyoxyethylene (POE) and alkyl chains is mostly broken, and the residual order in the chain is lost gradually by the increase in temperature of the mesophases. Based on the analysis of NMR line width, a qualitative picture was drawn concerning the molecular motion of the surfactant in various phases. No NMR signal is observed for the solid phase because surfactant molecules are highly immobilized. In the H-1 and L-a phases, the POE chain of the surfactant molecule can undergo segmental motion to some extent, whereas the motion of the alkyl chain is rather restricted. The molecular motion of the surfactants in the V-1 phase is quite rapid and is close to that in the liquid phase despite the highly viscous nature of that phase. The motion of the POE chain in the I-1 phase is somewhat restricted compared with that in the liquid phase probably due to the enhanced hydration of the POE chain in the spherical micelles constituting the I-1 phase.