The decomposition of the lithium hydride isotopologues (6LiH, 6LiD, 7LiH and 7LiD) and their propensity to accommodate non-stoichiometry is predicted using atomistic modelling methods. Significant differences exist between the reaction enthalpies of the isotopologues with respect to the hydrogen isotope, whilst negligible differences exist with respect to the incorporated lithium isotope. The calculated defect concentration identifies the vacancy formation energy for LiH is significantly lower than for LiD at a defined temperature. Further, the temperatures of complete decomposition to lithium metal for LiH and LiD are reported as 959 K and 999 K, respectively, a 40 K difference of potential use in breeder blanket concepts. The predicted decomposition of LiH is in good agreement with the experimentally observed value of 962 K. Defect concentrations at the decomposition temperature for all isotopologues are similar, despite the changes in temperature.