Bimetallic lanthanide (Ln: Eu or Yb)-Ni/SiO2 catalysts prepared by the use of dissolution of lanthanide metals in liquid ammonia have been studied by infrared spectroscopy for dinitrogen adsorption. The infrared spectra were measured at 133-300 K using Ln-Ni/SiO2 obtained when the Eu or Yb metal dissolved in liquid ammonia reacted with 20 mass% Ni/SiO2 in different ratios. Infrared spectra for Eu-Ni/SiO2 showed absorption bands at 2336, 2265, 2254 and 2227 cm(-1) at 133 K, which disappeared upon evacuation. The adsorbed state was found to be all molecular from the isotope shift using N-28(2) and N-30(2). The bands at 2254 and 2227 cm(-1) of them were assigned to new adsorbed dinitrogen species resulting from synergetic interactions between the europium and nickel metal. The concentration of adsorbed dinitrogen on Eu-Ni/SiO2 varied markedly with the Eu/Ni ratios, and particularly, it increased in the region of high Eu content. Upon introduction of ytterbium onto nickel, new bands at 2254 and 2226 cm(-1) similarly appeared. However, the dependence of dinitrogen adsorption as a function of Yb content in Yb-Ni/SiO2 was somewhat different from that for Eu-Ni/SiO2. The effects of lanthanide on the surface of Ln-Ni/SiO2 were discussed in connection with the variation in catalytic properties.