Hydrogen interaction with ruthenium surfaces is a complex phenomenon that can play an important role in catalytic reactions such as ammonia synthesis. In this study, H-2/D-2 isotopic homomolecular exchange has been used to characterize four different catalytic surfaces, namely Ru/AC(0), Ru/AC(1), Ru-Na/AC(0) and Ru-Na/AC(1). They are composed of 2%Ru or 2%Ru-4.5%Na supported on an active carbon with (AC(0)) and without surface groups (AC(1)). The AC(1) carbon (1162 m(2) g(-1)) is obtained by treatment of AC(0) (960 m(2) g(-1)) in N-2 at 900 degrees C. Ru/AC(0) and Ru/AC(1) catalysts are prepared by impregnation of the supports with aqueous solutions of Ru(NO)( NO3)(3). Na-promoted catalysts are then prepared by impregnation of Ru catalysts with NaOH solutions. An overall picture of the surface mobility phenomena on the four different catalytic surfaces has been described. It has been demonstrated that the presence of Na promoter in Ru-Na/AC0 and Ru-Na/AC(1) catalysts inhibits the spillover of H atoms from the Ru particles to the AC surface. However, for non Na promoted catalysts, the extension of the H spillover phenomenon depends on the amount of oxygen groups present on the AC surface. Likewise, the formation of a ruthenium hydride during the reduction treatment is suggested for catalysts promoted or not by Na. Finally, this work shows how the kinetics of the H-2/D-2 isotopic exchange reaction may be related to the surface electron density of the catalyst, which allows us to better understand the effect that both promoter and support have on the metal particles. Published by Elsevier B.V.