Lanthanide metals (Ln = Eu and Yb) dissolved in liquid ammonia and reacted readily with reduced Co or Ni powders to form lanthanide metal overlayers (Ln-Co or Ln-Ni). The surface properties of Ln-Co and Ln-Ni prepared from Co and Ni precursors with different particle sizes (15-28 nn) were characterized by the hydrogenation of propene. The hydrogenation behaviors varied markedly with changes in metal particle sizes of the Co and Ni precursors and the lanthanide content. The hydrogenation activity decreased with lanthanide content, passed through a minimum and increased with increasing content. The active surfaces of parent cobalt and nickel were gradually covered with the lanthanide metals and simultaneously synergetic interactions between the lanthanide and 3d-transition metals occurred, indicating the formation of hydrogen-absorbing surface intermetallic phases. This synergy showed a tendency to appear in the region of higher lanthanide contents with decreasing average metal sizes of the Co and Ni precursors.