We explored the hydrogen storage kinetics of Pd-Mg composites upon addition of different carbonaceous spillover agents (activated carbon and a wide spectrum of carbon nanotube types). We found that the hydrogen (loading or release) kinetics is strongly dependent on the nanocarbon morphology and configuration (e.g., length, diameter and Pd distribution). We therefore define a figure of merit quantifying the de/hydriding performance of previously reported systems and the system investigated in the present study. It demonstrates that the fastest kinetics is obtained for our Pd-decorated carbon nanotubes having the largest diameter. We found a clear structure-function relation between the spillover agent properties and the Mg de/hydriding rates, which could be applied in replacing the heavy and expensive transition metal catalyst by lightweight nanocarbon additive. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.