We have developed a new adiabatic calorimetry system for enthalpy of gas absorption/adsorption process. The sample cell is designed to be extremely small as a calorimeter cell with a resistance thermometer; mass: 4.34 g, inner volume: 1.06 cc. A thin stainless steel tube is connected at the lid of the cell to introduce a gas. A gas handling system was also constructed to introduce a gas slowly and measure the amounts of the absorbed/adsorbed gas precisely via a pVT method. As the first work on this system, we measured the enthalpies of H-2/D-2 absorption into Pd nanoparticles with an average diameter of ca. 8 nm. The obtained values are Delta H-abs (H) = -(18.15 +/- 0.87) kJmol(-1) and Delta absH(D) = -(18.17 +/- 0.38) kJmol(-1) at 298 K. These values are considerably smaller than those of bulk Pd. The isotope effect are discussed semi-quantitatively on the basis of the potential energy surfaces at the H/D sites.