The internal friction (IF) has been measured as a function of the H (D) content in the disordered and ordered Pd₃Mn system between 90 and 400 at frequencies in the range 180-5300 Hz. Relaxation processes due to stress-assisted motion of H (D) have been identified and their spectra of relaxation times, heights and temperatures determined as a function of H content and the state of long-range order. The main observations in the case of the alloy in the disordered state are an increase with n (n=H (or D)/(Pd+Mn) at.) of the height and width of the peak and a decrease of the peak temperatures. These effects have been attributed to progressive filling by H of higher energy I₅ sites (sites having five nearest-neighbour Pd atoms), which differ in terms of atomic arrangements in the second and more distant shells of neighbours. The partial molar enthalpy Delta H_H for H solution has also been determined both in the disordered and ordered states of the Pd₃Mn alloy. In the disordered state Delta H_H is found to increase rapidly with n starting from values of n as low as 0.01. The concentration dependence of Delta H_H as well as of the height of the IF peak reveal strong hydrogen-hydrogen repulsions.