Porous carbon fibers, whose active centers are formed by pores that are filled at least in part by carbon and/or metal and/or metal carbide, obtainable by carbonization of organic or inorganic polymers, the use thereof for the adsorption or separation of gaseous substances, in particular of CO.sub.2, and also a method for the production thereof. First, a spinning mixture containing polyacrylonitrile-based polymer A and an organic or metallo-organic polymer B is produced. Next, the spinning mixture is spun to form mixed fibers of polymer A and polymer B. The mixed fiber is stabilized by oxidation. This is followed by carbonization or graphitization of the mixed fiber under non-oxidizing conditions in such a way that the polymer B forms a carbon and/or metal and/or metal carbide residue of at least 22 wt %, the residue forming active centers.