The phase boundaries between the single phase regions α, β and the miscibility gap α+ β were determined for the PdH, PdD, and PdT systems in the temperature (T) range 40⩽ T⩽ 160° C and concentration (x= H atoms Pd atoms) ranges 0.01⩽ x⩽ 0.04, 0.54⩽ x⩽ 0.60. The limits of the α-phase in the temperature-concentration plane are shifted to higher concentration values with increasing isotope mass of the hydrogen atoms in Pd, whereas the phase boundary between the α+ β and β-regions is moved to lower concentration values in the sequence H, D, T. Although the experimentally observed phase boundaries shifts are small, the ratio of the phase boundary values for H, D and H, T can be described well by a simple harmonic model of the hydrogen atoms in Pd using the vibration frequencies determined by inelastic neutron scattering experiments.