Using a novel indirect nanoplasmonic sensing platform, we have measured hydrogen absorption/desorption isotherms in Pd nanoparticles with an average diameter from 1.8 to 8 nm at 30 degrees C. The isotherms are fully reversible at relatively low and high pressures while at intermediate pressures there is hysteresis related to the hydride formation and decomposition. The hysteresis shrinks with decreasing particle size and becomes negligible for particles smaller than 2.5 nm. This feature has been explained in terms of a diminishing contribution of lattice strain to the free energy of hydride formation as the particle size goes down. (C) 2010 Elsevier B. V. All rights reserved.