The effects of pressure and temperature on the reactivities of niobium cluster cations Nb-n(+)(n=4-16) toward molecular hydrogen and H-2/D-2 mixture were studied using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer combined with a laser-ablation/supersonic-jet external ion source. It was shown that the rate constants of the reactions of most Nb-n(+) clusters with the first H-2 molecule exhibit pronounced fall-off behavior in the total pressure range of 10(-6)-10(-4) Torr. For the Nb-n(+)+H-2/D-2 reactions, the H/D scrambling in the reaction products and the apparent kinetic isotope effects were also found to be strongly pressure-dependent. The observed phenomena are consistent with a reaction mechanism that involves a sequence of reversible stages of dissociative chemisorption of H-2 (or D-2) molecules on Nb-n(+) clusters. The fall-off effects can explain the apparent negative temperature dependence for the Nb-n(+)+H-2 reaction rate constants, the difference between the kinetic data obtained in FT-ICR and fast-flow reactor studies, and the observed enhanced kinetic isotope effects. Indirect evidence was obtained showing that the low reactivities of certain Nb-n(+) clusters (n=8, 10, and 16) toward hydrogen are due to activation barriers rather than low chemisorption energies.