The main aim of the current investigation is to compare the kinetics of devolatilization of wood spheres introduced at the top and close to the base of a fluidized bed gasifier. The experiments were performed in an externally heated batch reactor filled with olivine particles and fluidized with nitrogen. A known mass of wooden spheres with a diameter equal to 0.005 m was fed into the bed operated at 973, 1,073 and 1,173K. The molar flow rate of H2, CO, CH4 and CO2 as a function of time was determined with gas analyzers equipped with infrared or thermal conductivity detectors. A devolatilization model was suggested to explain the observed experimental increase of the gas yield when the particles were introduced close to the bottom of the bed. It accounts for the believed transient catalytic conversion of tar. It supposes that a catalytic decomposition of tar at the surface of olivine particles controls the rate of a three step catalytic mechanism. A comparison between experimental and calculated transient results of molar flow rate of formed gas species confirmed the reliability of the suggested model in both the examined circumstances, that is, when biomass was introduced at the top and close to the base of the bed. The results reveal that the olivine particles alter drastically the selectivity of the gasification, since only CO2 was formed due to the heterogeneous catalytic decomposition of tar.