The saturation levels of small nickel clusters Ni-n (n =2-20) with CO are determined. Modeling of the CO-covered clusters shows that in almost all cases saturation is governed by the geometrical sizes of the nickel clusters and the van der Waals size of the CO molecules. While electron counting rules, along with an assumed cluster structure, can predict saturation levels, the predictions generally overestimate the levels based on geometry. In the case of the smallest clusters (n less than or equal to 13), CO adsorption appears to cause changes in structure to more open ones that result in increased adsorption, giving coverages close to the electron counting rule predictions for the bare cluster geometries. Larger clusters, with internal metal atoms, seem to be more resistant to structural changes. Comparisons with earlier studies of nickel cluster positive and negative ions are made and the significance of a short reaction time scale on the nature of the reaction products is discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)00906-5].