The equilibrium sorption, inverse gas chromatography (dynamic sorption), optic microscopy, X-ray diffraction and visible light scattering methods were used to investigate the structure of poly(ethylene oxide) (PEO) blended with poly(vinylacetate) and poly(siloxanes) as matrices of polymer solid electrolytes. It was shown that the melting point (Tm) and the degree of crystallinity in such PEO-blends were lower than in pure PEO. The Gibbs energy of mixing and the compatibility parameter (χ12) were obtained for PEO-blends. In the vicinity of Tm all the systems were thermodynamically compatible, however at the ambient temperature they were microgeterogenous and compatible only in the narrow range of PEO-modificator contents. This must be taken into consideration while creating solid polymer electrolytes based on polymer blend matrices.