Reduction in hydrogen at 773 K of gallium-modified HZSM-5 zeolite prepared by incipient wetness impregnation results in complete Substitution of acidic hydroxyl groups. The modification of the zeolite with gallium suppresses dealumination of the hydrogen form due to substitution of protons by gallium species. When the reduction at 773 K is followed by evacuation, coordinatively unsaturated Ga+ ions are formed. These species can be reversibly oxidized by nitrous oxide at 673 K. Alternatively, these Ga+ ions adsorb molecular hydrogen at 4 A lower temperatures resulting in several types of hydride species. The latter species are completely decomposed only in vacuum at relatively high temperatures (773 K). We propose that the oxidation of gallium upon cooling of reduced samples to room temperature in hydrogen can be attributed to an oxidative addition of H-2 resulting in the formation of gallium dihydrides. (C) 2004 Elsevier Inc. All rights reserved.