The pyrolysis kinetics of biomass and coals are investigated by the thermogravimetric method. The influences of the initial sample mass, the heating-rate, the final temperature and the pretreatment of biomass are determined. The kinetics of the degradation process is described by an infinite number of parallel first-order reactions. The behaviour of the biomass during pyrolysis is similiar to the well-known behaviour of coal. The theoretical model fits well the experimental results based on biomass. The pyrolysis of the pretreated biomass is more difficult to describe because the biomass is devided in at least two fractions. Furthermore there is no interdependence during copyrolysis of biomass and coal.