The simultaneous measurement of both the relative electrical resistance and the equilibrium hydrogen and deuterium pressure as a function of composition of Pd - H and Pd - D systems have been carried out at temperatures between 273 and 323 K at pressures up to about 3.3 MPa. The relative resistance, , in the two-phase region for the absorption processes shows a very small and almost linear increase with increasing H(D) content, especially for the Pd - H system, compared to the larger changes previously observed by the electrolysis method. The resistance behaviour is quite similar to the shape of p - c isotherm relationships. The relative resistance increments per unit change of H(D)/Pd content at 298 K, , in the two-phase region are about 1.5 and 2.1 times larger for the Pd - H and Pd - D systems, respectively, compared to the changes in the relative lattice parameters with H(D)/Pd content, , within the two-phase region, where is the lattice parameter of H(D)-free Pd and r is the atom ratio. On the other hand, the resistance increment in the single solid solution phase and single phase, except for the higher-H(D)-content region, is significantly larger compared to the changes of the lattice expansion due to dissolved hydrogen and deuterium. Thus, the variation in resistance with hydrogen and deuterium content in the two-phase region may be mainly associated with an incoherent formation of hydride within the phase. The relative resistance for the subsequent desorption processes from the absorption up to about 3.3 MPa at 298 K in both Pd - H and Pd - D systems exhibits almost the same maximum as that of the absorption processes, i.e. at about and at about , and then the values decrease gradually with decreasing H(D) content up to the phase boundary composition; on entering the two-phase region, the values remain almost constant, i.e. for the Pd - H system and for the Pd - D system. This large hysteresis of resistance can be attributed to the creation of `lattice strain deformations' accompanied by dislocation formation from hydride (deuteride) formation and by further highly dissolved hydrogen and deuterium in the phase region.