A thermobalance reactor was used to evaluate the reactivity of poplar wood during gasification under operating conditions specific for the Hynol process where biomass is gasified at 30 atm and 800 degrees C with a hydrogen-rich gas recycled from methanol synthesis. The gasification involved a rapid devolatilization and pyrolysis reaction of the volatile matter in biomass and a slow reaction of residual carbon with the process gas. Nearly 86% of 1/8-in. poplar particles and 90% of sawdust were converted into gas products by a feed gas corresponding to the Hynol process (66% H-2) in 60 min. About 4% additional conversion of the 1/8-in. particles was obtained when gasification time was extended from 60 to 150 min. Gasification rate and biomass conversion were strongly affected by reaction temperature and particle size. The conversion was proportional to the partial pressures of H-2 and steam in the feed gas. A kinetic model was developed to correlate the experimental data and quantitatively express gasification rates and biomass conversion as functions of reaction time. The activation energies for the rapid and slow reactions were estimated to be 3.8 and 34 kcal/g-mol, respectively.