The phase behavior of a surfactant-like ionic liquid, N-hexadecyl-N-methylpyrrolidinium bromide (C16MPB), was studied in both water and a room temperature ionic liquid, ethylammonium nitrate (EAN). Polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) measurements were employed to investigate the phase behavior of the two systems and to determine which lyotropic liquid crystalline (LC) phases were formed. With increasing C16MPB concentration, an isotropic solution phase, a hexagonal (H-1) phase, and a cubic phase (V-2) are all present in either EAN or H2O. The structural parameters of the H-1 phase were calculated from SAXS patterns, which show the structural changes as a function of the amount of C16MPB. The rheological results reveal that the H-1 phase constructed by C16MPB in EAN displays a typical Maxwell behavior, whereas the H-1 phase formed by C16MPB in water shows a gel-like behavior, unlike traditional cationic surfactants. POM and differential scanning calorimetry (DSC) results demonstrate that the lyotropic LC phase in EAN has a higher thermal stability than that formed in H2O, which may be important to extend the applications of the LC phase.