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We present accurate experimental measurements of the lifetimes and term values of the metastable rovibrational f-symmetry levels of the I 1 Å g state of HD. These levels lie above the second dissociation limit of hydrogen. The lifetimes were obtained directly from the observation of the time-dependent decay of the fluorescence. These levels decay by direct fluorescence and by predissociation after tunnelling through the potential energy barrier of the I 1 Å g state. We present ab initio results for both of these decay processes for H 2 , HD and D 2 . The theoretical calculations provide an explanation of the rotational dependence of the observed lifetimes for HD, the lack of rotational dependence for D 2 , and the approximately equal lifetimes for N ¼ 1 for HD and D 2 . The calculated lifetimes for the metastable levels of H 2 explain why no experimental data is yet available for the parent isotopomer. However, the use of recently updated Ho¨nl-London factors with published ab initio electronic transition moments reveals a larger than expected discrepancy with experiment for those levels of HD and D 2 for which emission is the dominant decay mechanism.