We have characterized the structure and phase behavior of dioctadecyldimethylammonium bromide (DODAB) aqueous dispersions by using conventional and high sensitivity nano differential scanning calorimetry, microscopy, cryogenic transmission electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and electric conductivity measurements. Special attention has been paid to the formation and transformation of the subgel phase. An almost pure subgel can be obtained in the dilute region (below 7.5 mM), while an almost pure coagel phase can be obtained in the concentrated region (above 6.7 wt %). We found that unilamellar vesicles were spontaneously formed in the subgel phase of a 5 mM DODAB dispersion. Infrared spectroscopic data reveal that the only significant change during the gel to subgel phase transition is the ordering in the lipid alkyl chain packing. That is, the head and tail parts of the DODAB molecules change nonsynchronously upon the gel to subgel transition, and the subgel phase is triggered only by the change of the lipid tail part We propose that the morphological change (from curled membranes in the gel phase to unilamellar vesicles with faceted surface in the subgel phase) is coupled to the change of alkyl chain packing state during the gel to subgel transition. Finally, a full picture of the phase transition sequences for the dilute and concentrated DODAB dispersions is given.