Interactions ot CO with the sodium-precovered Ag(110) have been studied with X-ray photoelcctron spectroscopy (XPS) and ultravilet photoelectron spectroscopy(UPS). It is shown that there are two weakly chemisorbed molecular states on the Na/Ag(110) at low temperature. As the annealing temperature increasing, only one state regains which is associated with the existence of Na. The further experimental evidences show that this state is a necessary precursor to the dissociated state.