The growth and reactivity of surface alloy films of Sm/Ru(001) have been investigated by XPS and TPD techniques. A monolayer of Sm is formed on Ru(001) surface at 500 K. Beyond this coverage, surface alloy phase is highly favored due to interdiffusion process. The chemisorption of CO on this interface is both molecular and dissociative in nature. A new molecular CO species associated with Sm2Ox is detected in our studies. © 1998 Elsevier Science B.V. All rights reserved.