A detailed model is developed in order to describe the pyrolysis of biomasses in fluidised bed reactors. Preliminarly the physical and kinetic phenomena involved in the process are described. This work illustrates die level of detail the model for the particles must have in order to give reliable information and the way the model can be solved and coupled with the kinetics that take place outside the particles. A sensitivity analysis has supplied the parameters which influence relevantly the yields for different biomasses.