The deoxygenation of benzaldehyde has been investigated over gallium-modified ZSM-5 catalysts. In the absence of H-2, Ga/HZSM-5 catalyzes benzaldehyde decarbonylation resulting in benzene and CO. The active sites for this reaction are the strong Bronsted acid sites. In the presence of H-2, the main product is toluene. It is believed that Ga cationic species (Ga+/GaH2+) generated during H-2 reduction can promote the hydrogenation/hydrogenolysis reactions that give toluene and water. In the absence of H-2, toluene can only be observed in transient experiments when the Ga/HZSM-5 catalysts are reduced. It is suggested that the GaH2+ species generated under H-2 play an important role in the hydrogenation/hydrogenolysis. However, they readily decompose to Ga+ in the absence of H-2. The addition of water to the feed modifies the catalytic activity and selectivity of Ga/HZSM-5 catalysts. On the one hand, water generates additional Bronsted acid sites from the reaction of extra-framework Ga with chemisorbed water (GaO(OH)) and with defect hydroxyls of the zeolite framework (GaOHSi). These additional sites enhance the production of benzene but decrease the production of toluene, due to a decrease in the density of reduced Ga cationic species. (C) 2009 Elsevier Inc. All rights reserved.