The dispersion and activity of carbon-based ruthenium catalysts for ammonia synthesis were examined. Barium was used as a promoter. Parallel XRD and extensive chemisorption (H-2, O-2 and additionally CO) studies of a series of unpromoted ruthenium/carbon materials showed that a more developed texture (porosity and surface area) of the carbon support results in a significant increase in the ruthenium dispersion. It was found that the presence of ultra small mesopores (diameters<3 nm) in the support results in the formation of ultra fine ruthenium particles. Barium influences the adsorption of the three reactant gases: (i) chemisorption of oxygen increases slightly (15-20%); (ii) hydrogen chemisorption increases significantly (up to 100%); (iii) carbon monoxide chemisorption drops to a negligible value. Barium is supposed to decorate the ruthenium particles and to retard the migration of carbon-hydrogen species onto the metal surface. The promoted ruthenium catalysts exhibit extremely high activities in ammonia synthesis. Under the experimental conditions (p=6.0 MPa, T=673 K, x(NH3) = 8% H-2:N-2=3:1) the reaction rate referred to the number of Ru surface atoms (TOF) was by an order of magnitude higher than that of a commercial fused iron catalyst. The TOF values are about constant for large Ru crystallites (diameters>3 nm) but they decrease, very likely, for smaller ones.