The development of proton exchange membranes as electrolytes for polymer electrolyte fuel cells operating at intermediate temperatures has been achieved combining two approaches: the preparation of a cross-linked polymer and the formation of covalent organic/inorganic hybrids. A covalently crosslinked sulfonated polyetheretherketone with elevated degree of sulfonation (DS ) 0.8) was prepared by reaction with HSO3Cl (SOPEEK). No degradation of PEEK was detected upon sulfonation, but only cross-linking via sulfonic groups with formation of SO2 moieties was observed. The extent of ramification was calculated by analyzing the 1H nuclear magnetic resonance (NMR) spectra of the products and resulted in 20% of the total amount of -SO3H groups present in the polymer. The solubility of the -SO2Cl precursor in organic solvents allowed easy carrying out of functionalization reactions in homogeneous conditions by lithiation and subsequent reaction with SiCl4, thus introducing covalenly linked silicon moieties (SOSiPEEK) at the ratios 1:0.25 and 1:0.50 per monomeric unit. The products were characterized by 1H and 13C NMR, attenuated total reflectance Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry.