The incorporation of silica nanostructures in polymer membranes is beneficial for the chemical, mechanical and the life time stability of high temperature proton exchange membrane fuel cells. Since the properties are closely related to the morphology and chemical composition of the constituents, their detailed investigation is of importance. In this paper transmission electron microscopy is utilized to study cross-linked silica nanoparticles in phosphoric acid doped polybenzimidazole membranes, which are formed by an in-situ sol-gel reaction using varying reaction parameters. The results show that by an additional heating step before membrane casting, several hundred nanometer large, ellipsoidal silica particles are formed in the membrane. Small, elongated nanoparticles with sizes in the range of about 20 am are present in all samples independent of the heat treatment. In-depth energy dispersive X-ray spectroscopy and electron energy loss spectroscopy analysis proved the homogeneous distribution and composition of the amorphous silica nanoparticles and revealed that no enrichment occurs at the interface between nanoparticles and polymer. The ex-situ membrane investigations reveal a higher chemical and mechanical stability for membranes containing the large, ellipsoidal particles. However, the membrane with only smaller silica nanoparticles shows a better degradation behavior than the membrane with large particles. (C) 2015 Elsevier B.V. All rights reserved.