The size dependencies of the hydrogen-Storage properties in polymer-coated Pd nanoparticles with diameters of 2.6 +/- 0.4 and 7.0 +/- 0.9 nm were investigated by a measurement of hydrogen pressure-composition isotherms. Their storage capacities per constituent Pd atom in the particles decreased with decreasing particle size, whereas the hydrogen concentrations in the two kinds of nanoparticles were almost the same and 1.2 times as much, respectively, as that in bulk palladium after counting zero hydrogen occupancy on the atoms in the first surface layer of the particles. Furthermore, apparent changes in hydrogen absorption behavior with decreasing particle size were observed, that is, a narrowing of the two-phase regions of solid-solution and hydride phases, the lowering of the equilibrium hydrogen pressure, and a decrease in the critical temperature of the two-phase state. By analyzing the isotherms, we quantitatively determined the heat of formation (Delta H-alpha ->beta) and the entropy change (Delta S-alpha ->beta) in the hydride formation of the nanoparticle. Delta H-alpha ->beta and Delta S-alpha ->beta for the 2.6 +/- 0.4 nm diameter Pd nanoparticle were -34.6 +/- 0.61 kJ(H-2 mol)(-1) and -83.1 +/- 1.8 J(H-2 mol)K--1(-1), whereas for the 7.0 +/- 0.9 nm diameter Pd nanoparticles the values were -31.0 +/- 1.8 kJ(H-2 mol)(-1) and -67.3 +/- 5.1 J(H-2 mol)K--1(-1), respectively. These quantities gave us a prospective picture of hydrogen absorption in Pd nanoparticles and the peculiarities in the formation of a single nanometer-sized hydride.