The coupling between VV and VT transitions in H 2(2) + D 2(0) → H 2(1) + D 2(1) and D 2(2) +H 2(0) → D 2(1) + H 2(1) at high collision energies is investigated by use of the solution of the Schrodinger equation of motion. The coupling is sufficiently important that the energy exchange processes cannot be described by the VV mechanism alone.