The reactions of acetaldehyde were investigated on the surfaces of CeO2, 3 wt% Pd/CeO2, 3 wt% Co/CeO2, and 3 wt% Pd-3 wt% Co/CeO2 by temperature programmed desorption (TPD) and infrared spectroscopy (FT-IR). The surface and bulk compositions of these catalysts were studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and CO chemisorption. XRD patterns indicated that the fluorite structure of CeO2 was maintained in all catalysts (calcined and H-2-reduced) and that there were no apprediable differences in particle dimension between CeO2 alone (96 Angstrom) and metal/CeO2 catalysts (84-98 Angstrom). Reduced catalysts were investigated by XPS : Ce (3d) spectra indicated the precence of Ce3+ cations, evidenced by V’ and U’ peaks at 885.0 and 903.7 eV, together with Ce4+ species. Co 2p spectra indicated the presence of Co2+ cations, evidenced by the Co 2p(3/2) line at 781.0 eV (and its satellite at about 788 eV) and Co 2p(1/2) at 797.0 eV (and its satellite at about 804 eV). Pd metal was detected on Pd/CeO2 and Pd-Co/CeO2. Several reactions of acetaldehyde were observed; they can be classified as oxidation, reduction, or carbon-carbon bond formation reactions. Oxidation to acetates and reduction to ethanol were observed on all catalysts; acetate species were identified by their IR bands at 1560-1540, 1451, 1400, 1343, and 1020 cm(-1) at room temperature, and ethanol was observed to desorb at ca, 400 K during acetaldehyde TPD. Several other adsorbed species were observed (on hydrogen reduced catalysts) by IR : acetyl, fingerprinted by a 1684 cm(-1) band on metal/CeO2, eta(2) (C, O) acetaldehyde (bands at 1220-1268, 1175-1156, 950-940 cm(-1)) on Pd/CeO2 and Pd-Co/CeO2, and CO adsorbed by its carbon to metal (Pd, Co) and by its oxygen to reduced Ce, giving rise to bands at 1730, 1739, and 1750 cm(-1). Four C-C bond formation reactions were observed : (1) beta-aldolization to crotonaldehyde and crotyl alcohol (most prominent on CeO2 alone), (2) acetate ketonization to acetone and CO2 on CeO2 and Co/CeO2, (3) acetyl reaction with methyl species to give acetone on Pd/CeO2, Co/CeO2, and Pd-Co/CeO2 (this acetone desorption was coincident with propane desorption on Pd/CeO2 and Co/CeO2), (4) reductive coupling of two molecules of acetaldehyde to butene and butadiene (on CeO2 and Co/CeO2). Pd-Co/CeO2 adsorbed four times more CO than did Pd/CeO2 or Co/CeO2 and was the most active for acetaldehyde conversion.