The enthalpies for the reaction of gaseous hydrogen and deuterium with palladium have been measured as a function of [H]/[Pd] and [D]/[Pd] atomic ratios up to 0.865 and 0.85, respectively, in the temperature range 298 - 194.5 K using a differential-heat-flow low-temperature calorimeter; pressure - concentration isotherms have been measured simultaneously. The calorimetric two-phase enthalpies for the Pd - H and Pd - D systems at 298 K are and , and the corresponding entropies determined from the calorimetric enthalpies and the absorption plateau pressures are and . These - and -values are in agreement with the previously reported values and also with values determined in the literature from van't Hoff plots of the plateau pressures.

The calorimetrically determined enthalpies for solution in the -phase regions of both systems become less exothermic in an almost linear fashion with increasing H(D) content, independently of temperature. At the same H(D) content, the -values for the Pd - H system are slightly more exothermic than the -values for the Pd - D system. Enthalpies of almost the same magnitude are obtained from the desorption data. The corresponding entropies, for solution in the -phase region of both systems have a tendency to decrease gradually with increase in H(D) content independently of temperature; however, there is no marked difference between the magnitudes of the -values of the two systems. The dependences of the calorimetric enthalpies for hydrogen (deuterium) solution on H and D contents in the ranges and can be expressed by the temperature-independent relations , and . The variations in -values with H(D) content are in agreement with that calculated from van't Hoff plots of the previously reported relative chemical potentials of hydrogen and deuterium at low temperatures. The present calorimetric data determined by gas-phase measurements do not offer support for the generation of any `excess heat' up to [D]/[Pd] = 0.85, beyond that which is expected from the chemical reaction because the values determined calorimetrically are almost the same as those derived from van't Hoff plots.