This study reports the lyotropic phase behavior of two poly(ethylene oxide)-b-poly(butadiene) diblock copolymers and their cross-linking in the mesophase under retention of the mesoscopic order. The lyotropic phase behavior in water was characterized by polarized light microscopy and small-angle X-ray scattering (SAXS) in the concentration range from 0 to 100 wt % and in a temperature range between 20 and 100 degrees C. Depending on polymer composition and concentration micellar, hexagonal, lamellar, and cubic phases are found. Their ranges as well as pronounced coexisting phase regions were determined. Several of these mesophases were cross-linked via gamma-irradiation to form mesostructured hydrogels. It is shown that the cross-linked polymer gel essentially maintains the parental lyotropic order, as proven by SAXS, polarized light microscopy, and transmission electron microscopy (TEM). TEM enables imaging of the polymer gel structure and thereby the visualization of the Liquid-crystalline mesophase morphologies in themselves. The lyotropic mesophases as well as the lyotropic gels were used as templates for the synthesis of mesoporous silica, which is expected to give a negative solid copy of the ordered soft matter structure. The influence of the different templates on the silica structure formation is discussed.