Bimetallic alloy surfaces exhibit quite different adsorption properties from properties of the elemental metal surfaces. The differences in the behaviour can be explained in terms of electronic (ligand effects) and structural (ensemble composition effects) concepts. In this work, Auger electron spectroscopy-thermal desorption spectroscopy measurements were used to characterise the surface of two polycrystalline Co50Pd50 and Co37Pd63 (at.%) alloys exposed to some oxygen-containing probe molecules, i.e. CO, O-2 and N2O at 300 K. In addition, X-ray photoelectron spectroscopy was used to elucidate the variation of electronic properties of Co and I'd upon alloying. The studied alloy surfaces proved to be active in dissociative chemisorption of O-2 and N2O as well as in the chemisorption of molecular CO. The desorption measurements revealed that CO molecules were less tightly bound to these surfaces than to the individual metals. This observation suggests an electronic interaction between Co and Pd atoms in the alloy, as evidenced by X-ray photoelectron spectroscopy measurements for core and valence bands. NO decomposition process resulted only in molecular N-2, which promptly desorbed, and O atoms, which remained at the surface, poisoning further N2O adsorption by a site blocking mechanism. Amount of desorbing oxygen molecules significantly depended on the exposure of both O-2 and N2O adsorbates and the Co surface concentration (an ensemble composition effect). (C) 2004 Elsevier B.V. All rights reserved.