The thermal decompositions of sugarcane bagasse and waste-wood samples are studied using thermogravimetric analysis. Assuming the addition of three independent parallel decompositions, these corresponding to three pseudocomponents linked to the hemicellulose, cellulose, and lignin, the weight loss associated with the pyrolysis process is simulated. First, an irreversible first-order reaction model is assumed for each pseudocomponent. Results show that the model-simulated curves do not fit well to the experimental data. Consequently, a further kinetic study is carried out for the pure lignin (Kraft alkali lignin), which demonstrates that the pyrolysis of lignin is better described by a third-order reaction rate law. The reformulation of the lignin kinetic model, and its subsequent implementation in the summative model (for the third pseudocomponent), has allowed one to reach a good agreement between simulated and experimental data.