The pyrolysis of lignocellulosic materials (pine and eucalyptus woods and pine bark) was previously studied by thermogravimetry with linear temperature programming. The results were successfully modelled by a kinetic scheme consisting of three independent first-order reactions of three pseudo-components. The first and the second pseudo-components correspond to the more-reactive fractions of hemicellulose and cellulose, respectively, and the third includes lignin and the remaining fractions of the carbohydrates. In the present work, the isothermal pyrolysis of lignocellulosic materials (in the temperature range 450-700 K) was simulated using the previously determined kinetic parameters, yielding their corresponding relative reactivities. The reactivity of each pseudo-component was also studied, and it was found out that the first pseudo-component was completely converted in a very short time at medium and high temperatures, while complete conversion of the second required high temperatures. The reactivity of the third pseudo-component is moderate, even at the highest temperatures. Based on these observations, a method to determine pyrolysis kinetics from isothermal thermogravimetric experiments was developed, whereby the kinetic and composition parameters corresponding to pseudocomponents 3, 2 and 1 are successively and independently determined, from high, medium and low temperature experiments, respectively. The method was validated using the isothermal thermogravimetric curves simulated for the three materials.