Activated carbon fibers were prepared by chemical activation of hemp fibers with phosphoric acid at different carbonization temperatures and impregnation ratios. Surface properties of the activated carbons fibers were significantly influenced by the activation temperature and the impregnation ratio. An increase of either of these parameters produced a high development of the porous structure of the fibers. Activated carbon fibers with apparent surface area of 1350 m(2)/g and mesopore volume of 1.25 cm(3)/g were obtained at 550 degrees C with an impregnation ratio of 3. The activated carbon fibers presented a high oxidation resistance, due to the presence of phosphorus compounds on the carbon surface. The oxidation resistance results suggest that C-O-PO(3) and mainly C-PO(3) and C-P groups act as a physical barrier, blocking the active carbon sites for the oxidation reaction. (c) 2008 Elsevier Ltd. All rights reserved.