Abstract The nuclear quantum effects (NQE) on the geometries and energies of the HeH + and He 2 H + complexes and their hydrogen isotopologues are investigated with the recently developed APMO/MP2 code. This code implements the Nuclear Molecular Orbital Approach (NMO) at a MP2 level of theory for electrons and quantum nuclei. In a first investigation of NQE on HeH + , we have observed a reduction in the bond distance as the mass of the hydrogen isotope is increased. Our calculated bond distances are in good agreement with experiment. We have also studied the H/D/T isotope effects on the geometry, total energy and stabilization energy of HeHHe + . We have also determined the relation between nuclear mass and delocalization and found that the lighter the nucleus the more delocalized it is. Our results demonstrate the importance of including nuclear quantum effects in these systems. To our knowledge, these are the first reported results on isotope effects on the HeH + and He 2 H + complexes using a NMO method. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010