The phase behavior of aqueous mixtures of a nonionic surfactant, hexaethylene glycol dodecyl ether (C12E6), 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 T-X phase diagram of this system was constructed from the DSC results. Comparison of the phase diagram with those previously obtained for C12E7 and C12E8 demonstrates that with the decrease in the polyoxyethylene (POE) chain length, the region of L-alpha phase expands toward higher temperature, whereas the H-1 region shrinks toward lower temperature, This behavior is well explained in terms of the change in the critical packing parameter of the surfactant molecules associated with the change in the POE chain length. FT-IR results indicate that the ordered conformational structure of both the hydrocarbon and POE chains is kept after the transformation from solid to mesophases. The order-to-disorder transformation of the conformational structure of the chains takes place rather gradually within mesophases with the increase in temperature, This behavior of the chain melting associated with the temperature rise is common to aqueous mixtures of POE-type nonionic surfactants. C-13 NMR line widths depend strongly on the phase structure formed by this mixture system: quite broad signals are observed for H-1 and L-alpha phases, whereas the signals for V-1 phase are rather sharp. According to the two-step model for NMR relaxation in aqueous surfactant systems, the line broadening observed in H-1 and L-alpha phases is attributed to the anisometry of the molecular assemblies in these phases.