We have studied the structure of nafion membranes by small-angle neutron scattering in the q range of 7 10(-3) Angstrom (-1) to 0.4 Angstrom (-1). External conditions were varied: counterions, electrolyte concentration, temperature, and pretreatment of membranes. The nanostructure of Nation exhibits a sensivity that depends on the parameters considered: changing counterions and concentration leads to a reversible local reorganization of the hydrophilic cavities, whereas changing temperature and pretreatment leads to a irreversible reorganization on a larger scale. The mean variation of the radius of the hydrophilic cavities versus these parameters is about 10%. The interface between hydrophobic and hydrophilic phases appears to be sharp except when the temperature exceeds 60 degreesC. (C) 2001 John Wiley & Sons, Inc.