A standard chemisorption procedure has been set up for the determination of Ru dispersion in Ru/C catalysts. Pulse chemisorption of oxygen was carried out at 0 degreesC, after having proved that no corrosive chemisorption phenomena are present. An average chemisorption stoichiometry Ru*/O = 1.0 was experimentally determined through measurements on Ru black. The procedure was applied to the investigation of promoted and unpromoted Ru/C catalysts for ammonia synthesis, supported on two different carbon supports. The main factor influencing Ru dispersion showed to be Ru loading, while the addition of even large amounts of Ba-Cs-K promoters has practically no influence. It is also briefly discussed how such results can help in elucidating several aspects of the behaviour of Ru/C as catalyst for ammonia synthesis. (C) 2003 Elsevier Science B.V. All rights reserved.