Fast pyrolysis of two typical biomasses (legume straw and apricot stone) in a free-fall reactor was studied. The pyrolysis behavior of the biomass is strongly relevant to its chemical composition. Under fast heating and pyrolysis conditions as in the free-fall reactor, an interaction of the biomass pyrolysis with the in situ steam reforming of the pyrolysis intermediate products occurs, which leads to more hydrogen-rich gas yield. The total ratio of CO and H-2 in the gas product reaches 65.4 mol% for legume straw and 55.7 mol% for apricot stone, respectively. Effects of sample particle size and pyrolysis temperature on the fast pyrolysis of the biomass were also studied. (C) 2004 Elsevier B.V. All rights reserved.