A membrane containing 7-23 nm polystyrene sulfonate domains was prepared using a reactive block polymer precursor and evaluated for its ammonia selectivity. The ammonia permeability is over 20.1 x 10(-17) kmol m/(m(2) s Pa) (600 barters) at 25 degrees C and 2 x 10(5) Pa (2 bar) pressure; the selectivity for a mixed gas feed containing ammonia and nitrogen is over 90, about the same selectivity that is inferred from the pure gas feeds (the "ideal selectivity"). Variations in the sulfonated phase domain size and the degree of crosslinking in the membrane give permeabilities which increase with the average ammonia pressure and retain their selectivity at higher pressure. The results may have value in the separation of ammonia produced by the Haber-Bosch process. (C) 2009 Elsevier B.V. All rights reserved.