A catalyst reactor coupled with analytical pyrolysis gas chromatography/mass spectcoscopy (Py-GC/MS) was used to carry out online analysis of the product vapors from the fast pyrolysis of Miscanthus x giganteus, Scots pine, and mahogany. Pyrolysis was carried out in both an inert atmosphere of He gas and in a highly reducing atmosphere of H-2. Significant changes in the vapor compositions were achieved with the use of H2 as the carrier gas. The most notable of these were the increases in the hydrocarbon compositions. The roles of ZSM-5, Ni/ZSM-5, MGM-41, and of Ni/MCM-41 catalysts on the compositions of the pyrolysis vapors were investigated. Lower amounts of the higher molecular weight phenolic compounds and larger amounts of the lighter phenols were observed in the presence of Ni supported on ZSM-5 and MCM-41. This effect was more evident for the 10% than for the 2.5% Ni loadings. Overall, the results both from the use of H2 as the carrier gas and from all the catalysts demonstrates significant improvements in the composition of the vapors. However, this resulted in the lowering of the quantities of condensable products.