Catalytic steam pyrolysis of biomass was performed in a bubbling fluidized-bed reactor at 450 degrees C, and the effects of silica-supported transition metals (Ni and V) on product yields and compositions have been investigated. Both metals seem to be catalytically active and altered the liquid composition. An interesting finding is the in situ reduction of the supported nickel oxide to metallic Ni during the pyrolysis reactions, which can enhance H-transfer. Vanadia-containing catalysts show higher selectivity in reduction of carboxylic acids and ketones. An increased aldehyde content, especially for the bimetallic Ni-V catalyst, suggests that selective deoxygenation can take place via the Mars van Krevelen (MvK) mechanism. Ni catalysts showed activity for aromatics formation, while both metals showed selectivity in producing phenols instead of catechols. Assessment of the catalytic performance indicates that both metals could be interesting candidates for incorporation in other support materials and evaluation of the derived modified catalysts in biomass steam pyrolysis.