A laser ablation fast-flow reactor apparatus, equipped with a laser ionization time-of-flight mass spectrometer detector, has been used to investigate the reactivity of gas-phase Nb, with isobutane; Absolute second-order rate coefficients k((2)) for the removal of Nb-n by isobutane were measured at 279, 300, and 371 K in a continuous flow of He carrier gas held at 1 or 2 Torr total flow tube pressure. The rate coefficients for a given cluster species were found to be pressure independent, and became larger with increasing temperature. A gradual increase in k((2)) with increasing cluster size has been observed. This increase is simply explained by a direct correlation of k((2)) with the hard-sphere collision cross section. Pseudo-first-order kinetic plots for all cluster reactions with isobutane were linear with no bimodal character evident, indicating an insensitivity to structural forms of Nb-n. Activation energies have been estimated, and their values were found to be close to zero for all Nb-n cluster reactions with isobutane. The small magnitudes of k((2)) relative to k(hs) have been attributed to a reaction mechanism involving a "tight" transition-state complex. Product masses, after the reaction of a bare Nb cluster with one or more isobutane molecules, clearly show dehydrogenation to be an active process. A pseudo-first-order kinetic model for a consecutive bimolecular reaction scheme has been solved for the Nb-2/isobutane system, and is in good agreement with the experimental data.