Thermal analysis can be used for the behaviour of plants during heating and most particularly in the risk prevention of forest fire. Cellulose, holocellulose and lignin are the most important compounds determinated by analytical procedures in six mediterranean vegetals: Acacia dealbata, Pinus halepinsis, Quercus coccifera, Rosmarinus officinalis, Thymus vulgaris, Ulex parviflorus. TG-DTA is applied to the powders obtained after drying (110°C), pounding and sifting. The rate of pyrolysis about 300°C allows a semi-quantitative estimate of the cellulose present in the vegetal and its flammability.