Novel proton exchange membranes were solvent-cast from DMF solutions of the terpolymers poly[ (MA-alt-S)-co-AMPS], containing hydrophobic phenyl and reactive hydrophilic carboxylic and organo-sulfonic acid fragments with different compositions, and PEGs with different molecular weights and amounts. These membranes were formed as a result of physical (via H-bonding). and chemical (via PEG) cross-linking. The structures of membranes were confirmed by FT-IR and H-1- and C-13 NMR spectroscopy. Mechanical and thermal properties swellability, and proton conductivity of these membranes were significantly affected both by the chemical composition of the terpolymers (mainly the AMPS content) and also the cross-linker (PEG) molecular weight and content in the final form of the membranes. It was concluded that the membranes prepared by using the terpolymer with an AMPS content of 36.84 mol-% and PEG with a molecular weight of 1450 and with an initial PEG content of 30 wt.-% are the most suitable ones for fuel cell applications.