Order-to-disorder transition of micellar aqueous solution of hydrophobically modified ethyleneglycol oligomer, poly(ethyleneglycol) monododecylether (C12E25) with addition of poly(ethyleneglycol) (PEG) has been investigated by means of linear-viscoelastic measurements, X-ray scattering, and differential scanning calorimetry. We have revealed the followings. Added PEG having a chain length similar to that of micellar corona PEG can be incorporated into the micellar lattice of C12E25 aqueous solution with maintaining the body-centered-cubic lattice structure. The transition mechanically observed as the sol-gel transition is a thermodynamical first-order transition of disorder-to-order. The transition temperature decreases with increasing concentration of guest PEG. The stability of micellar lattice is discussed in terms of guest chain localization in the lattice. (C) 2003 Elsevier Ltd. All rights reserved.