Carbon fibers made from phenolic resins or from isotropic pitch can be used as filamentous adsorbent materials. The aim of this note is to compare the change in microporosity of carbon fibers activated in steam and in carbon dioxide respectively. For the experimental conditions used, steam activation leads to a more pronounced shift from micropores to mesopores than activation by CO2 and as a result, a smaller micropore volume is obtained at a given level of activation.