In situ X-ray diffraction (XRD) and gravimetric hydrogen uptake measurements of d similar to 23 nm spherical PdH(x) particles have been studied in the temperature and pressure range of 323 < T < 428 K and 0 < P < 10 bar, The Pd particles were protected from sintering with a hydrogen-permeable carbon coating. While only containing similar to 300-1000 atoms, the Pd particles were found to exhibit the same fcc structure and lattice constant as the bulk. our isothermal studies show that, with increasing x, these highly crystalline PdHx nanoparticles also exhibit a complete transformation from the dilute a solid solution phase to the more concentrated beta hydride phase. However, we observed that the character of the alpha-beta phase transition in these nanoparticles is very different from that in the bulk. Indeed, the hydrogen uptake isotherm exhibits a noticeable positive slope in the alpha + beta co-existence region. Furthermore, we also observed a noticeable narrowing of the a + [3 co-existence region (delta x) in the nanoparticles. Also, a significant suppression of the critical temperature T(c) for the phase boundary was observed: T(c)(nano) approximate to 430 K while T(c)(bulk) approximate to 570 K. These results signal a significant change in the thermodynamic behavior of very small hydride nanoparticles that may be common to many other nano-scale metal hydride systems as well. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.