While operating in an H-2-O-2 polymer electrolyte fuel cell (PEFC), a membrane electrode assembly containing a pore-filling electrolyte membrane composed of a porous cross-linked polyethylene (CLPE) substrate filled with poly(acrylamide tert-butyl sulfonic acid) (PATBS) showed excellent fuel cell performance at ambient pressure and a relatively low operating temperature (T = 60 degrees C). The conductance during operation was also determined. The CLPE-PATBS membrane showed a relatively high open-circuit voltage of similar to 1.0 V, despite the thinness of the membrane, indicating that the membrane suppresses H-2 fuel crossover. (C) 2004 The Electrochemical Society.