The 5μm absorption region of an isotopically pure sample of diborane 1 0 B 2 H 6 has been analyzed using high-resolution (0.0033cm - 1 ) Fourier transform spectra. A set of precise experimental rotational-vibrational energy levels has been obtained for three vibrational states, the 13 1 fundamental and two combination states, 5 1 15 1 and 9 1 15 1 . These energy levels have been fitted using Hamiltonian matrices which take into account various interactions with nearby lying dark states. However, given the complexity of the resonance scheme and the number of interactions and dark states, it was not possible to reproduce the observed energy levels to within their experimental uncertainty although considerably improved results have been obtained compared with previous studies.