The microstructure of semidilute mixed solutions of poly(ethylene oxide) (PEG) and sodium dodecyl sulfate (SDS) was characterized by the combination of cryogenic-temperature transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). Cryo-TEM provides direct images that elucidate the basic microstructural building blocks of the system, with which an accurate physical model can be constructed for scattering data interpretation. SAXS then provides the desired quantitative information, which is not accurately available from microscopy. This experimental approach has been most successful in proving the existence of thread-like SDS micelles in the presence of high NaCl concentration, and their transformation to spheroidal micelles upon polymer addition. Preliminary data on a more strongly interacting system, namely gelatin/SDS, are also presented.