Stern-Gerlach magnetic deflection measurements were performed on Ni-n and NinCO (n=8-18) in a molecular beam experiment. Adsorbed carbon monoxide was found to reduce the magnetic moments of most nickel clusters in this size range. For Ni-8, the reduction was similar to5 bohr magnetons, while smaller reductions were observed for the other clusters. The present results are in qualitative accord with previous experiments and calculations of the magnetization quenching effects of CO on both nickel nanoparticles and thin nickel films. The magnitude of the changes in cluster magnetic moments of Ni-8, Ni-15, and Ni-18 were larger than those predicted by the local quenching (bond number) model, indicating that the CO-induced perturbation to cluster electronic structure in these clusters is distributed among many nickel atoms.