This paper highlights the importance of micropore size of carbonous materials in the storage capacity of hydrogen. A study is made of a series of carbon fibers with different burn off, activated by a new procedure in which supercritical CO2 is used, as well as of several activated carbons. The best storage results at 77 K and 1 bar corresponded to the activated carbon fibers. The maximum value obtained for these materials was 2.86wt%, significantly higher than that obtained previously by other authors. The study shows that storage increases rapidly with the degree of activation of the fiber and is closely linked to the micropores. Micropores of around 0.6 nm are those responsible for the greater increase in storage. This study confirms that activation with supercritical CO2 may lead to microporous solids with enormous capacity for adsorbing H2.