The geometrical structure of Ni-39 is probed via molecular adsorption of nitrogen on its surface. Nitrogen uptake patterns are determined at various reaction temperatures, and the results are interpreted in terms of the number and nature of nitrogen binding sites on possible structures. It is found that an adsorbate-induced isomerization occurs at partial nitrogen coverage, but that at saturation a reverse isomerization returns the cluster to its initial structure. The two lowest energy structures calculated by Wetzel and DePristo [T. L. Wetzel and A. E. DePristo, J. Chem. Phys. 105, 572 (1996)] are completely consistent with the observed saturation and isomerization behavior. The structures consist of atom caps packed around a central pentagonal bipyramid, and reflect a marked change from the octahedral structure determined earlier for Ni-38. [S0021-9606(98)70947-X].