The solution and liquid crystalline phases formed by dissolution of the dye Edicol Sunset Yellow (ESY) in water have been examined using optical microscopy, multinuclear NMR (H-1, H-2, C-13, Na-23), and X-ray diffraction. From the solution H-1 and C-13 spectra (particularly C-13) it is clear that the tautomeric form present in all these phases is the hydrazone, NH, structure, not the usually given azo, OH, form. Two chromonic mesophases occur: a nematic (N) phase at similar to 30-40 wt % and a hexagonal (M) phase at similar to 40-45 wt %. X-ray diffraction data show that the aggregates in the mesophases are single molecule stacks, with a typical spacing of similar to 3.5 angstrom, as expected for these systems. The NMR quadrupole splittings ((H2O)-H-2, Na-23) are similar to those observed for surfactant lyotropic mesophases, suggesting that there are no water molecules or counter ions that are tightly bound to the ESY aggregates. An unusual feature of the X-ray diffraction pattern of the mesophases is the occurrence of diffuse off-axis reflections at similar to 6.8 angstrom. It is proposed that these arise from a head-to-tail packing of the molecules within the stacks.