The dissociation process for CO adsorbed on Pd/W has been studied using a semiempirical molecular orbital formalism. The behavior of the bimetallic system has been examined comparing it with the pure metal surfaces. The different contributions to the dissociation barrier have been calculated and analyzed according to electronic structure arguments. Over Pd/W the CO dissociation is a very unlikely process, as the experimental evidence demonstrates, due to a lower population of 2 pi* molecular orbitals in comparison with Pd or W.