This paper deals with the gasification reactivity and kinetic modeling of biomass chars derived from a wide range of plant origins. For most chars studied, the gasification reactivity increased with increasing conversion or exhibited a maximum in high conversion range. Common theoretical models could not account for these peculiarities. A semi-empirical gasification kinetic model was proposed on the basis of modifying random pore model, which was found to be capable of describing the experimental data well over the entire conversion range for 14 biomass chars studied. It was found that the fitting parameters introduced in the modified random pore model predicted well the amount of the active potassium (K) in biomass chars. The results led to a reasonable conclusion that the maximum rate at high conversion range was mainly attributed to the catalytic effect of K. (c) 2007 Elsevier Ltd. All rights reserved.