Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nickel clusters were produced in a high intensity cluster ion source and thermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass-and charge-selected nickel clusters then passed through a Linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monoxide. The reaction products were then analyzed by a quadrupole mass-spectrometer, Using this technique, saturation limits for Ni-n(+) clusters with n=4-31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation was observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluster growth may be explained by a pentagonal sequence of structures for n=4-7, capping of the pentagonal bipyramid to buildup an icosahedron at Ni-13(+),and further capping of this icosahedron to form a double icosahedron at Ni-19(+). (C) 1997 American Institute of Physics.