An NMR study of the low-temperature adsorption of hydrogen on alumina and molybdenaalumina catalysts has led to the detection of at least two, and on extensively reduced molybdenaalumina catalysts, three, types of adsorbed hydrogen species. A single, averaged absorption line was observed originating from fast exchange between a small amount of strongly chemisorbed gas and the majority species which was molecular and much more weakly held. On alumina and unreduced molybdena-alumina the strongly chemisorbed species is also thought to be molecular, but evidence was obtained for dissociative adsorption on extensively reduced catalysts at low coverage. The signal width for the latter was that of the alumina surface hydroxyl groups; it remained constant with increasing coverage while the signal intensity increased. About 6 × 10 13 H 2/cm 2 were adsorbed at the knee of the curve, where averaging with the narrow band from the weakly adsorbed gas first became detectable. Interestingly, this broad signal disappeared when the extensively reduced surface was poisoned with about 3 × 10 13 O(atoms)/cm 2. NO also acted as a poison but was less selective. These studies provided a bridge between the present studies of the low-temperature adsorption of hydrogen and previous data for the hydrogenation of propylene around room temperature. Since both O 2 and NO are known to poison the catalytic hydrogenation at 21 °C, it was concluded that the site responsible for the low-temperature adsorption and dissociation of H 2 may be identified with those responsible for the catalytic process. Studies of the longitudinal relaxation process over the alumina catalyst showed that T 1 decreased (at constant coverage) with increasing adsorption temperature, as expected for relaxation by translational diffusion when ωτ ⪢ 1. At the same time, T 1 decreased (at constant coverage) as the extent of dehydroxylation increased, suggesting an interaction with 27Al (CUS). Intermolecular processes, e.g., collisions between adsorbate molecules, were not important in the relaxation mechanism as judged by the fact that T 1 remained constant when D 2 was substituted for part of the H 2 at constant pressure. The signal from reduced molybdena-alumina catalysts could not be saturated. For the alumina catalysts, at constant coverage the linewidth decreased as the adsorption temperature increased, but increased with extent of dehydroxylation. For molybdena-alumina the linewidth increased with the extent of reduction. Since only ortho-H 2 is detected by NMR, the kinetics of the first-order approach to equilibrium could be monitored in the adsorbed phase. Since the total adsorption was measured volumetrically, and the ortho-H 2 by NMR, the separation factor could be calculated in the adsorbed phase. As expected, values obtained (~2.0) indicated an enrichment of ortho-H 2 above the equilibrium value in the gas phase.