A method of determining an average degree of saturation with ammonia (X) of a solid ammonia storage medium porous or not and capable of ad- or absorbing and desorbing ammonia reversibly in a storage container is described. A part of the volume (Vcon) of the container is occupied by gaseous ammonia of a pressure (p) and defines a free volume (Vfree)). Ammonia flows out of the container with a flow (f). n pairs ((fi, pi), (Fi, pi)) of flows (fi), or accumulated flows (Fi), and pressures (pi) are sampled at a sequence of points of time (ti), i=1 . . . n and n≧2; an estimate volume value (Vfit) on the basis of the sampled pairs ((fi, pi), (Fi, pi)) is determined;and the degree of saturation with ammonia (X) is determined by applying a predetermined relationship (Rel) between a plurality of estimate volume values (Vfit) and a plurality of values of the average degree of saturation with ammonia (X) to the determined estimate volume value (Vfit).