Abstract Phase behavior in (NH 4) 2HPO 4 and (ND 4) 2HPO 4 has been studied using ac calorimetry and dielectric constant measurements. An unexpected reverse shift has been observed in the phase transitions temperatures at T 1=174 K and T 2=246 K for the proton compound to 147 K and 229 K, respectively, for the deuteron counterpart. A third phase transition near T 3=416 K shows no effect when substituting H for D. The phase transitions at low temperatures are discussed in terms of ammonium reorientations at T 1 and T 2. Since a high generation of defects are induced in both compounds above room temperature, as indicated by electrical conductivities measurements, we claim that, at 416 K, the observed first-order phase transition is triggered by the coupling of these defects with the lattice distortion. The change of crystal structure from monoclinic to orthorhombic symmetry that exhibits the 416 K transition influences the proton transport characteristics and, mainly, the mobile proton density which changes discontinuously through the phase boundary at T 3. However, there might be some dominant interaction for the transition to take place which has a negligible dependence on the mobile D + (H +) ion mass.