A series of ruthenium catalysts deposited on graphitized carbon and promoted with barium, caesium or both Ba and Cs have been studied in ammonia synthesis. Under experimental conditions (90 bar, H-2:N-2 = 3:1, 400 degreesC, 10% NH3), the reaction rate over the co-promoted catalyst (9.1 wt.% Ru in Ru + C) was found to be higher than those over singly promoted specimens, the overall effect from Ba + Cs in Ba-Cs-Ru/C being almost as high as the sum of individual effects from Ba (Ba-Ru/C) and Cs (Cs-Ru/C), respectively. The co-promoted Ru catalysts, especially that of high ruthenium loading (23.1 wt.% Ru) were shown to be significantly more active in NH3 synthesis than the conventional fused iron catalyst (KMI, H. Topsoe). The oxygen chemisorption studies have shown that the amounts Of O-2 taken up by the Cs-containing samples (Cs-Ru/C, Cs-Ba-Ru/C) are considerably larger than those for Ru/C and Ba-Ru/C, thus indicating caesium to be in a highly reduced, most likely zero valent form, when operating under ammonia synthesis conditions. It is suggested that barium ((Ba + O)(adlayer)) is located on the ruthenium surface and it acts as a structural or electronic promoter. Caesium (Cs-0) is suggested to be localised on the carbon surface: the Cs promotion occurs at contact points between Ru and the Cs atoms adsorbed on carbon ("hot ring promotion"-electronic). (C) 2003 Elsevier Science B.V. All rights reserved.