The geometrical structures of Ni-46, Ni-47, and Ni-48 are investigated by the chemical probe method using molecular nitrogen as the probe molecule. Nitrogen saturation levels are determined and found to be consistent with cluster structures derived from two geometries, one having face-centered-cubic (fcc) packing and the other fcc/hcp (hexagonal-close-packed) layered packing. The fcc-based structures provide the best overall agreement with the experimental measurements. The saturation levels are inconsistent with icosahedral or decahedral packing, a structure found for Ni-49 and larger clusters in earlier experiments. The change in cluster structure between Ni-48 and Ni-49 (most likely from fcc to icosahedral packing) is clearly evident in the nitrogen adsorption experiments at both low and high coverages, as well as with other reagent molecules. (C) 2001 American Institute of Physics.