Catalytic fast pyrolysis is a method for converting lignocellulosic biomass into renewable aromatics and olefins. Water is a byproduct of this reaction and is also present in the biomass feed. As the water partial pressure is increased from 0 to 212 Torr (0 to 28 kPa), there is an increase in furan conversion from 43.8 to 84.8 % over ZSM-5. The CO2 and propylene yields also increase from 0.7 to 16.4 % and 2.9 to 44.9 %, respectively, as the water partial pressure increases. Water partial pressures in an industrial catalytic fast pyrolysis reactor should be within the range of water partial pressures used in this study. These results demonstrate that the presence of water promotes hydrolysis reactions in the gas-phase conversion of furanic pyrolysis vapors over zeolite catalysts.