Reactions of tungsten clusters with molecular nitrogen have been investigated by using a fast-flow reactor equipped with a laser vaporization cluster source and time-of-flight mass spectrometer detector. Absolute rate coefficients are reported for reaction of W-n clusters in the range n=4-26, at temperatures 277, 300, and 370 K in He buffer gas at 1 and 2 Torr pressure, For smaller clusters with n<15, complexes with N-2 are formed with binding energies near 16 kcal mol(-1), and act as precursors to dissociation of N-2 on the clusters. A sharp jump in the binding energy occurs at a cluster size of 15 metal atoms, and may signal the onset of atomic as opposed to molecular binding of Nz on the cluster. It is suggested that the change in reactivity at n=15 is correlated with a structural transition of the clusters, from relatively close-packed to more open structures. The reactivity of tungsten clusters with N-2 is compared with that of molybdenum clusters and tungsten metal surfaces.