The vibrational dynamics of the H / 5 complex and its deuterated isotopomers H 4 D / , H 3 D / 2 , H 2 D / 3 , HD / 4 , and D / 5 are investigated using a model Hamiltonian which is based on the assumption that on the potential energy hypersurface the barriers for the internal rotation motions are infinitely high except for the practically free propeller-like motion. According to our previous studies on H / 5 , the propeller-like rotation essentially does not interact with the remaining vibrations and is therefore neglected in the present calculations. Within the framework of the adiabatic approximation the resulting eight-dimensional vibrational problem is separated into two smaller subproblems which are solved numerically applying the same scheme as previously. The calculations are performed using a new extended potential energy function which also provides a reliable description of the interactions between the degenerate stretching motions and the remaining vibrations. As in our previous calculations, the highfrequency fundamentals obtained for H / 5 are in good agreement with their experimental counterparts, whereas the reliability of the present results for the low-frequency motions is considerably improved as a result of the appropriate description of the relevant interactions. Predictions of the vibrational energies of the other deuterated isotopomers are made on the same accuracy level. The zero-point energies derived from the present calculations are believed to be accurate enough for a quantitative determination of the binding energies of the different isotopomers.