Microporous polypropylene hollow fibers can be coated with a 1.9 mu m layer of perfluorosulfonic acid polymer (’Nafion’), to give membranes selective for ammonia. The permeability, of the order of 10(4) barrers, is comparable with that obtained previously for thick flat perfluorosulfonic acid membranes, though the permeance is much higher. The selectivity for ammonia over hydrogen can be as high as 500:1 at room temperature; it is much higher for ammonia over nitrogen. This selectivity drops as the temperature rises. These composite membranes will be commercially valuable if they can be made to withstand the high pressures characteristic of ammonia synthesis.