Present study concerns to the thermal degradation of two carbon fibre/epoxy composites, which differ by their volume fractions in carbon fibre (56 and 59 vol%), investigated in cone calorimeter (under atmospheric condition with a piloted ignition). In order to study the influence of the carbon fibre amount on the composite thermal decomposition, the cone calorimeter external heat flux was varied up to 75 kW m(-2). Thus, main parameters of the thermal decomposition of two different composites determined were: mass loss, mass loss rate, ignition time, thermal response parameter, ignition temperature, critical heat flux, thermal inertia and heat of gasification. As a result, when carbon fibre fraction decreases from 59 to 56 vol%, an increase of the thermal parameters was observed: 14-18 kW m(-2) for critical heat flux, 370-435 kW s(1/2) m(-2) for thermal response parameter, 2.25-5.07 kW(2) s m(-4) K-2 for thermal inertia and 16-18 kJ g(-1) for gasification heat. By analysing the mass loss rate evolutions, a four phases thermal decomposition mechanism is proposed. In the first phase, epoxy resin is cracked to form low molecular weight gaseous species and epoxy-derived compounds. For two next phases, the combustion of epoxy resin and liquor monomer solvent is observed that induces the formation of carbon char. In the last phase, char oxidation and carbon fibre decomposition are identified. Further, during the composite decomposition process, thermal behaviour of solid matrix is changed from a thermally thick material to a thermally thin one when sample is exposed at high external heat flux above 20 kW m(-2). Copyright (c) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.