A theoretical study of the interaction of CO with a Au surface has been made based on the properties of molecular orbital wavefunctions for cluster models of the surface. A main objective is to use the information about the interaction, including dependence on the morphology and on the charge of the Au cluster, to help understand the catalytic activities of Au surfaces and nanoclusters. The work to be described is a first step toward this objective and some unexpected and surprising properties of the CO-Au interaction will be presented. Detailed analyses have been made to determine the important features of the electronic structure of the bond between the CO and the Au. The consequences of this bonding for the shift of the CO stretching frequency will be analyzed in terms of individual contributions to the chemisorption bond. The chemisorption bond for CO/Au has similarities to the bonding of CO/Cu but there are also important differences. In particular, the dp to 2p ^* back-donation is considerably larger for Au-CO than for Cu-CO.