Infrared absorption due to adsorbed dinitrogen on Ru metals supported on Al2O3 and MgO is observed at 2214 and 2168 cm(-1), respectively, under N-2 atmosphere at 160-300 K. The different wavenumbers were obtained depending on the reduction temperature of the samples. On cesium nitrate doping, the bands shifted to lower frequencies such as 2095 cm(-1) for Ru-Cs+/Al2O3 and 1910 cm(-1) for Ru-Cs+/MgO. These frequencies are related to the electronic states of Ru particles, which are affected by the supports and the promoters. These surface natures are discussed in connection with the ammonia synthesis activities. The heat of adsorption of dinitrogen on 10 wt % Ru/MgO is estimated to be 14 kJ mol(-1). Through the infrared absorption of adsorbed dinitrogen, the change of the surface Ru state by the oxidation-reduction cycles was studied. Ru particles became more dispersed on MgO surfaces by the oxidation-reduction cycles. Active ammonia catalysts were characterized by an FTIR through the adsorbed dinitrogen.