Thin and thick palladium overlayers were deposited onto clean and oxidized niobium surfaces, Thermally programmed desorption of carbon monoxide, chemisorbed at room temperature on these Layers has been investigated. Qualitatively similar results, i.e., weak interaction of CO with thin Pd overlayers and a stronger one with thick overlayers both for Pd/Nb and for Pd/NbOx/Nb systems have been obtained.