Addition of hydrophobic guest molecule (C12H26) into the nonionic surfactant/water system (C16E7/D2O) system modifies the phase behavior between the lamellar and column phases. The distribution of the added guest molecule in the microstructure and the interfacial structure is examined by using a small angle neutron scattering technique. The characteristic length and the interfacial thickness of the microstructure are deformed in order to optimize the distribution of the guest molecules. The obtained experimental results including the existence of the double gyroid and rhombohedral phases are in agreement with those of the simple model of diblock copolymers. (C) 2003 American Institute of Physics.