Four different one-phase regions: two liquid phases (L and L-3) and two liquid crystalline phases (L-alpha and V-2) of the MO/NMP/water system, were studied by SAXS, PGSE-NMR, and rheology at 20 degreesC. The location of the dramatic increase in solubility of MO, in binary NMP/water mixtures, can be explained in terms of the presence or absence of free water, as a consequence of strong NMP(water)(2) complex formation in the binary NMP/water system. A direct effect of this complexation is aggregate formation at a certain region within the L, phase. The microstructure of the L-3 phase can be modeled as bicontinuous according to the theory of interconnected rods when formation of NMP(water)(2) complexes are taken into consideration. The swelling of the sponge phase correlates well to predicted swelling laws and characteristic lengths, as found in other sponge phases. The cubic phase region of the ternary system consists of at least two different cubic phases, with (Pn3m) over bar and (Ia3d) over bar symmetry of the space group, respectively. At higher NMP contents it seems as if the (Pn3m) over bar symmetry of the space group transforms into (Im3m) over bar.