We implemented a model at the atomistic level to simulate metal-hydrogen (Me-H) pressure-composition equilibrium isotherms and we applied it to Ni and Pd, both at the bulk and in nano-sized particles. We used potentials of the embedded atom type (EAM) to describe the interaction between atoms [Daw MS, Baskes MI. Embedded-atom method: derivation and application to impurities, surfaces, and other defects in metals. Phys Rev B 1984; 29(12): 6443-53; Angelo JE, Moody NR, Baskes MI. Trapping of hydrogen in nickel. Model Simul Mater Sci Eng 1995;3:289-307] and we performed Monte Carlo simulations to calculate the isotherms. For the bulk the simulations are sensitive to the interatomic potentials chosen. The tested potentials for Pd-H were not adequate since they did not exhibit the characteristic plateaux observed experimentally. The Ni-H potentials predicted a clear transition but at lower hydrogen pressures. For Ni and Pd nano-sized clusters our simulations predict enhanced hydrogen solubilities and vanishing plateaux when compared to the bulk. For both types of nanoparticles H atoms were segregated to the surface, but in Ni the effect was stronger. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.