Hydrogen storage of platinum-MWCNT composites is studied using the Sievert's volumetric equipment. The adsorption performed at 1.67 MPa and 298 K has resulted in a storage capacity of 2.9 +/- 0.3 wt%, 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 PtH2 and PtH4, we found that these surface atoms belong to up to 15 closed-shells. (c) 2007 Elsevier B.V. All rights reserved.