In this study, the effect of recycling the non-condensable gases (NCG) in the catalytic pyrolysis of hybrid poplar using FCC catalyst was investigated. A 50 mm bench scale fluidized bed reactor at 475 degrees C with a weight hourly space velocity (WHSV) of 2 h(-1) and a gas recycling capability was used for the studies. Model fluidizing gas mixtures of CO/N-2, CO2/N-2, CO/CO2/N-2 and H-2/N-2 were used to determine their independent effects. Recycling of the NCG in the process was found to potentially increase the liquid yield and decrease char/coke yield. The model fluidizing gases increased the liquid yield and the CO2/N-2 fluidizing gas had the lowest char/coke yield. The C-13-NMR analysis showed that recycling of NCG increases the aromatic fractions and decreases the methoxy, carboxylic and sugar fractions. Recycling of NCG increased the higher heating value and the pH of the bio-oil as well as decreased the viscosity and density. (C) 2012 Elsevier Ltd. All rights reserved.