Fast pyrolysis of four kinds of biomass (legume straw, tobacco stalk, pine sawdust and apricot stone) was conducted in a free fall reactor. Interest is focused on hydrogen-rich gas production. The experimental results verify the occurrence of the in-situ steam reforming of tar, the steam gasification of char and the water-gas shift reaction with the primary pyrolysis of the biomass at higher heating rate in the free fall reactor. These reactions influence greatly the products' distribution and dry gas compositions in fast pyrolysis, especially at higher temperature. The pyrolysis is mass and heat transfer controlled for biomass particle size of above 0.20 mm but kinetically controlled in the case of particle size smaller than 0.20 mm. Biomass composed of higher cellulose and hemicellulose favors hydrogen-rich gas production in fast pyrolysis than that composed of higher lignin. The pyrolysis characteristics of each type of biomass can be explained according to its chemical compositions. (c) 2006 Elsevier B.V. All rights reserved.