One of the most interesting strategies for improving hydrogen storage capacity at room temperature in porous solids is the inclusion of catalytically-active metal nanoparticles to provide the spillover effect in MOF materials. However, there is a great controversy in the adsorption literature about this topic due to the low reproducibility of results and the lack of knowledge about this mechanism. In the present work, 5-9 nm Pd nanoparticles were homogeneously dispersed in a ZIF-8 material by a simultaneous liquid impregnation and chemical reduction with hydrazine. After verifying the structural and textural properties, the so obtained Pd/ZIF-8 material was tested in H-2 and D-2 adsorption-desorption analysis in order to determine its final hydrogen adsorption properties as well as to understand the involved mechanism in the hydrogen retention over the sample. Pd/ZIF-8 material showed a completely reversible hydrogen storage capacity at room temperature 30% higher than that of the pristine ZIF-8, being only 3% related to PdH0.18 formation. These results provide a better knowledge of the hydrogen retention mechanism by spillover in Pd-doped MOF materials, being useful in the designing of new materials for hydrogen storage at room temperature. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.