This work is a comparative study of the variations in the structure of a carbon fibre activated with supercritical H2O and CO2 vs. steam and CO2 at atmospheric pressure (conventional process). The appearances of the external and internal surfaces, fibre size, external and internal mass loss, density, porosity, and fragmentation, etc. were measured and analysed. The results are explained by the different reactivity of the supercritical fluids compared with the steam and CO2 at atmospheric pressure. The results are coherent with a gasification model based on percolation theory, and special emphasis is given to fibre disintegration once the carbonaceous matrix porosity has reached a critical value. (C) 2015 Elsevier B.V. All rights reserved.