The segregation of bulk Al after the hydrogen desorption may be one of many reasons why there is the incomplete hydrogen re-absorption and the need for higher desorption temperature in subsequent desorption of HfCl(4)- and TiO(2)-doped NaAlH(4). In this work, we attempted to improve hydrogen storage capacity of 4 mol% HfCl(4)- and 4 mol% TiO(2)-doped NaAlH(4) by adding a co-dopant (graphite, activated carbon, and MCM-41) using ball milling technique. it was found that the co-dopants significantly affect the hydrogen desorption/re-absorption of the hydride. Results show that the hydrogen re-absorption capacity of HfCl(4)- and TiO(2)-doped NaAlH(4) added with the co-dopants increases 10-50% as compared with that without a co-dopant, and graphite seems to be the best co-dopant. Moreover, the hydrogen desorption temperature in the subsequent cycle of co-dopant doped samples decreases about 10-15 degrees C compared to the sample without a co-dopant. It could be explained that high porosity and large surface area of the co-dopants would decrease the segregation of bulk Al after the desorption and improve hydrogen diffusion in/out bulk of desorbed/re-absorbed samples. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.