The behaviour of biomass components (cellulose, xylan-representative of hemicelluloses-and lignin) was studied thermogravimetrically with linear temperature programming, under nitrogen and air. The results were compared and the pyrolysis kinetics of cellulose determined, assuming a first-order kinetic function. The thermal decomposition of xylan and lignin could not be modelled with acceptable errors by means of simple reactions. Thermograms were determined for pine and eucalyptus woods and pine bark, under inert (nitrogen) or oxidizing (air) atmosphere. The pyrolysis of these lignocellulosic materials was modelled with good approximation by three first-order independent reactions. One of these reactions is associated with the primary pyrolysis of cellulose, its parameters being previously determined and fixed in the model. The model parameters are the activation energies and pre-exponential factors for the pyrolysis of the remaining two pseudo-components and two additional parameters related to the biomass composition. A method to determine this composition was proposed. The results calculated in this way were compared with data from the literature and those determined thermogravimetrically in air, showing good agreement.