Thermodynamic properties (Delta H degrees(R), Delta G degrees(R)) of the radicals originating from the reactions of the compounds modeling building blocks of the polymer membranes used in low temperature fuel cell applications with hydroxyl radicals and oxygen molecules are studied in water environment and room temperature. All characteristics of the compounds are calculated using the UB3LYP DFT method with spin unrestricted orbitals. The Polarizable Continuum Model (PCM) is employed to model the solvation of species by water. A degradation mechanism for the nonfluorinated aromatic polymers sPEEK and PSU, which leads to the loss of sulfonic acid groups, is proposed. The process starts with the addition of the (OH)-O-center dot radical to aromatic rings. At the next step, an oxygen molecule is attached to the cyclohexadienyl radical forming various OH-aromatic-OO type radicals. These species can act as direct precursors for the loss of sulfonic acid group. Alternatively, they can transform into bicyclic and/or epoxy-type radicals, which are labile towards the detachment of the sulfonic acid group. The presence of water favors the detachment reactions significantly. The processes lead to decreased proton conductivity and contribute to the reduction of the membrane performance in PEMFCs and DMFCs. (c) 2005 Elsevier B.V. All rights reserved.