The hydrogen storage capacities of the two sandwich-type Cp2TM2 [Cp = cyclopentadienyl (C5H5); TM = V or Cr)] dimetallocenes were studied using first-principles calculations. According to the B3PW91 method in density functional theory (DFT), Cp2V2 and Cp2Cr2 could adsorb up to seven and six hydrogen molecules, respectively. The predicted hydrogen storage densities of Cp2V2 and Cp2Cr2 were 5.73 and 4.91 wt%, respectively. Additionally, because hydrogen binding energies bound by Kubas model were 0.39 and 0.43 eV/H-2, respectively, both the Cp2V2 and Cp2Cr2 dimetallocenes were proven to be favourable for reversible adsorption and desorption of hydrogen molecules under ambient conditions. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.