Dissociative chemisorption of nitrogen is the rate-determining step in the synthesis of ammonia. The nitrogen molecule donates a pair of electrons to the empty d-orbitals of the metal catalyst, which in turn donates the delectrons to the antibonding orbit& of the nitrogen [ 1, 21. The strength of adsorption and the extent of dissociation of nitrogen depend presumably on the extent of back-donation of the electrons from metal to nitrogen. Aika et al. [3-S] and various other investigators [g-12] have carried out extensive work on Ru catalysts using various supports and promoters in order to obtain better ammonia synthesis activity at atmospheric pressure. Very recently, it has been found that the addition of alkali ionic promoters, Cs and K, and some alkaline earth metals such as Ba [8], to the Ru supported on active carbon form effective catalysts. There has been renewed interest in identifying novel promoters and supports which increase the electron density on the metal. As a part of this process, the authors have recently studied the synthesis reaction on Cs-Ru and Cs-Ru-Ba catalysts supported on carbon-covered alumina and have reported the influence of the support and the Ba addition to it on the steady state activity [ 12, 151. In the present study the activity of the catalysts with increasing and decreasing reaction temperature has been determined. During these temperature cycles, a rate hysteresis is observed on these catalysts, just as Richard and Vanderspurt [ 131 observed on their triply-promoted Fe catalyst at 30 and 90 atm. Carbon-covered alumina (CCA) was prepared by the pyrolysis of ethylene (Matheson, USA) in Nz (IOLAR-1, India) at 600 “C over commercial y-A1203 (Harshaw, Al-3996 R, l&/25 BSS mesh). The flow rates of ethylene and nitrogen were 2 ml min’ and 15 ml min’ respectively per gram of alumina. The procedure followed was similar to that of Vissers et al. [ 141. The carbon