We have performed an XPS, TDS, ISS and LEED investigation on the adsorption of ruthenium carbonyl, Ru 3 (CO) 1 2 on a Co(0001) surface. The adsorption of the carbonyl is molecular at 120 K and its dissociation occurs at 300 K via desorption of CO molecules. The desorption of CO is completed above 500 K, after which XPS and ISS indicate an atomically clean bimetallic surface, and LEED reveals a hexagonal (1 1) pattern. The ISS and XPS measurements as a function of Ru coverage indicate the formation of a surface alloy. The lattice constant of the alloy shifts from bulk Co value to bulk Ru value during the Ru deposition. The surface alloy formation is discussed in terms of a Monte Carlo simulation.