We report molecular orbital Hartree-Fock and post-Hartree-Fock adiabatic calculations of HDO and HID, so that the isotope effect is accounted for. The isotope symmetry breaking of HDO is predicted for the first time. The isotope shift of the states of HD converging to the n=2 separated atom level is obtained exactly. Mulliken population analysis illustrates the symmetry breaking phenomena, from D-proportional toh to Cinfinity(D) in HD and from C-2v to C-1 in HDO. (C) 2003 Elsevier B.V. All rights reserved.