Disclosed is a self-supporting solid compacted composition of material comprising an ammonia storage material capable of reversibly absorbing and desorbing ammonia in its ammonia saturated state and compacted to such a density that it has of at least 70% of the single crystal density of said material in its ammonia-saturated state in admixture with one or more additive incapable of storing ammonia and having a thermal conductivity of at least five times the thermal conductivity of the ammonia storage material at a temperature of from at least −45° C. to 250° C., as well as a process for preparing it.