Degradation in air has been carried out of some woods and agricultural residues at heating rates of 20 K/min. The thermal behaviour has also been investigated with commercial products taken as representative of the main biomass components. Weight loss curves for woods and agricultural residues can be interpreted by a three-step reaction scheme. The first, very fast step, can be attributed to the degradation of all the main components, the very slow second stage to lignin and hemicellulose degradation, while the third is the combustion of the solid residual. The amount of volatiles released in the first stage is higher for woods (75 to 77% of the total solid against 50 to 60%) of residues) and the process is displaced towards higher temperatures and thus associated with higher activation energies (values of about 75 to 100 kJ/mol for woods and 72 to 78 kJ/mol for residues). In all cases, devolatilization rates are very slow and almost constant in the second stage, with small amounts of volatiles released (6 to 9% for woods and 5 to 16% for residues) and comparable activation energies (86 to 92 kJ/mol for woods and 83 to 87 kJ/mol for residues). Combustion of chars from agricultural residues again starts at lower temperatures and is described by activation energies of 73 to 80 kJ/mol (against 71 to 90 kJ/mol for woods).