The present paper describes the thermal conditions under which a solid particle (biomass) undergoes an endothermic decomposition caused by an external heat flux. The results derived from a mathematical model concern two extreme cases of conditions following the size of the particle (chemical and ablation regimes). The sensitivity of the results is studied as a function of several experimental parameters (particle size; heat transfer coefficient; heat source temperature) and chemical characteristics (kinetics and enthalpy). The sensible parameter governing the reaction temperature is the activation energy, The enthalpy has a minor effect except on heating rates. The reaction temperature is always much lower than the heat source temperature. In the chemical regime it is shown that after a simple heating phase, the temperature at which the reaction starts, varies between relatively close limits (ess than 80 K). A temperature stabilization is then observed as the decomposition proceeds in such a way that the reaction may be considered as quasi-isothermal mainly for high enthalpies. In the ablation regime, the particle shrinks rapidly at a constant velocity, the reaction occurring inside an external thin layer. The reaction temperatures and heating rates are always lower than in the chemical regime. Data derived from previous experiments on fast pyrolysis of wood (ablation conditions) are in good agreement with the predictions of the model (reaction temperature, ablation rate and ablation layer thickness). The results bring confirmation of the effect of fusion observed during the thermal decomposition of biomass.