A semifluorinated fatty acid self-assembles into aggregate in water and gelatinize water. The morphology of the hydrogel was observed by scanning electron microscope (SEM). Its aggregate structure and the properties have been investigated by small-angle X-ray diffraction (SA-XRD). Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The results confirm that the hydrogen bonds between the adjacent molecules with water as mediator are the primary cause of hydrogel formation. (C) 2002 Elsevier Science B.V. All rights reserved.