The selective hydrogenation of 1,3-butadiene on Pd(111) and Pd(110) model catalysts was studied at atmospheric pressure in the temperature range 298-373 K. In the initial stages of the reaction, 1-butene, trans-2-butene, and cis-2-butene were the main reaction products. After full 1,3-butadiene consumption, 1-butene was readsorbed and reacted to n-butane via hydrogenation and to trans/cis-2-butene via isomerization. Because hydrogenation was favored on Pd(111), Pd(110) exhibited a higher selectivity for butene formation. Increasing the reaction temperature accelerated both processes, and hydrogenation was favored at long reaction times. Addition of small amounts of CO drastically changed the selectivity on Pd(110); that is, the hydrogenation to n-butane was completely suppressed, whereas the hydrogenation to butenes and 1-butene isomerization to cis- and trans-2-butene were hardly affected. This can be rationalized by considering that CO reduces the hydrogen surface concentration to a level that is still sufficient for 1,3-butadiene hydrogenation and 1-butene isomerisation but too low for butene hydrogenation. In contrast, on Pd(111), catalytic activity basically vanished in the presence of CO traces. Possible reaction mechanisms are discussed. (c) 2005 Elsevier Inc. All rights reserved.