There is described a method and apparatus (100, 100′) for storing and delivering ammonia wherein at least two ammonia storage materials (1a, 2a) capable of reversibly adsorbing or absorbing ammonia having different ammonia vapor pressures are used. Ammonia storage material (2a) having a lower vapor pressure, which is only partially saturated with ammonia or void of ammonia, is brought into fluid communication with ammonia storage material (1a) having a higher ammonia vapor pressure to adsorb or absorb ammonia released from the ammonia storage material (1a) having a higher ammonia vapor pressure when the latter is higher than a pressure threshold. An automotive NOx treatment system (200) comprising such apparatus (100, 100′) is also described.