The changes in energy for (CF3)(2)CFOCF2CF2SO3H, a model for sidechains in perfluorosulfonated membranes, such as Nafion(R), as a function of chain dihedral angles have been analyzed by ab initio SCF-MO calculations. For the highest level calculations, B3LYP/6-311 + G*//HF/4-31G*, the barrier for rotation about a C-C bond is similar to 10 kJ mol(-1) and similar to 30 kJ mol(-1) about a C-O bond. Analytical expressions fitted to the values from the ab initio calculations for both rotations are presented. The effect of different solvents on the rotation barriers has been calculated via SCRF methods and shows the barriers to decrease for highly polar solvents. The possibility of intrachain hydrogen bonding to occur is outlined and discussed in relation to the ability of the membranes to rehydrate.