The deuterium desorption pressure-composition isotherms (PCIs) of ZrCo1-xNix-D-2 (x = 0, 0.1, 0.2 and 0.3) systems were generated in this study in the temperature range of 524 -603 K using Sievert's type volumetric apparatus. Thermodynamic parameters like enthalpy and entropy change for deuterium desorption reactions involved in the ZrCo1-xNix-D-2 systems were derived using the equilibrium pressure data of PCIs. In order to interpret the hydrogen isotope effect on the storage behaviour of ZrCo1-xNix alloys, the results obtained in the present study were compared with the earlier reported data on the ZrCo1-xNix-H-2 (x = 0, 0.1, 0.2 and 0.3) systems. This comparison revealed that these alloys show normal hydrogen isotope effect where the equilibrium pressure of D-2 is higher than that of H-2 at all experimental temperatures. Based on these observation, it is expected that at the ITER SDS operating conditions the equilibrium pressure of tritium, deuterium and hydrogen will follow the order: p(T-2) > p(D-2) > p(H-2) for these alloys. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.