Ligno-cellulosic biomass from different sources presents variable composition. The main aim of this work was to develop a method to predict the gas yields after flash pyrolysis (and tar cracking) at 950 _C in an Entrained Flow Reactor of any biomass from its composition in the three main components - cellulose, hemicellulose and lignin. For this approach to be successful, three conditions need to be met: (C1) Pyrolytic behaviour of celluloses from different biomasses is similar, as is hemicellulose and lignin behaviour. (C2) There is no interaction between the components. (C3) Extractives and ashes have no impact on the pyrolysis process. Two approaches were chosen to investigate the condition C1: (i) Celluloses, hemicelluloses and lignins of various sources were pyrolysed. Results show that hemicelluloses and lignins from different sources do not form the same quantities of gases. (ii) An attempt was made to identify the gas yields of ''theoretical components'' that are able to predict flash pyrolytic behaviour of any biomass. Results tend to show that this is not possible. The condition C2 is investigated by comparing the gas yields of the components taken separately and the gas yields of mixes of the components. Two types of mixing were carried out: simple mixing and intimate mixing. Results show that interactions occur between the components during flash pyrolysis. The condition C3 was not investigated here; it can nevertheless be concluded that the behaviour of a biomass during flash pyrolysis at high temperature cannot be predicted from its composition in cellulose, hemicellulose and lignin.