From the atomistic simulation of pressure-composition isotherms of hydrogen absorption of Pd nanoparticles we calculated thermodynamic properties and evaluated them as a function of particle size. In the particle range studied both |Delta H| and |Delta S| decreased with particle size towards the corresponding bulk values. H atoms were segregated to the subsurface of the particles forming a shell structure that may explain an initial plateau shown in the particles isotherms that is not present in bulk simulations. We used potentials of the embedded-atom type (EAM) to describe the interaction between atoms and we performed Monte Carlo simulations to calculate the isotherms. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.