The Li-promoted CO dissociation on flat and atomically stepped Ru surfaces has been investigated by thermal desorption spectroscopy (TDS) and infrared reflection-absorption spectroscopy (IRAS). Li-induced dissociation of CO is observed to be greatly enhanced on stepped Ru(109) compared to Ru(001) from the scrambling of isotopic CO molecules as well as from the intensity reduction of the CO stretching mode. Interestingly, on Ru(001), Li produces a highly red-shifted CO stretching mode at similar to 1820 cm(-1), implying the formation of Li,CO, complexes. On the other hand, the preferential adsorption of Li by the atomic steps on Ru(109) impedes the formation of such complexes. (c) 2006 Elsevier B.V. All rights reserved.