Hydrogen storage of platinum-MWCNT composites is studied using the Sievert’s volumetric equipment. The adsorption performed at 1.67MPa and 298K has resulted in a storage capacity of 2.9±0.3wt%, nearly 40 times higher compared to that of pristine. The enhancement is attributed to the interfacial diffusion of hydrogen from platinum to the nanotubes. However, this spillover operates only with the coordinately unsaturated platinum atoms on the particle surface as revealed from the atomic magic number-estimated nanoparticle storage capacity. By comparing the storage capacity of the composites with that of PtH 2 and PtH 4 , we found that these surface atoms belong to up to 15 closed-shells.