The paper discusses the use of a thermobalance reactor to evaluate the reactivity of popular wood in gasification under the operating conditions specific for the Hynol process (30 atm and 800 C). The gasification involved a rapid devolization and pyrolysis reaction of the volatile matter in biomass and slow reaction of residual carbon with the process gas. Gasification rate and biomass conversion were strongly affected by reaction temperature and particle size. The conversion was proportional to the partial pressures of hydrogen 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/mol, respectively.