The comparative thermal and chemical study was done on five mediterranean plants: Acacia dealbata, Quercus coccifera, Rosmarinus officinalis, Thymus vulgaris, Ulex parviflorus. Results were independent of the drying way of samples (110C, 60C or lyophilisation). The correlation between the maximum decomposition rate at about 300C and the content of cellulose was confirmed. The samples cut on the same foot of Ulex parviflorus, in spring, produced, for the thorns and sprigs, a minimum in cellulose content, and a maximum in lignine content. The results were inverse in winter.