The dynamics of propynal, HCCCHO and HCCCDO, have been investigated in an excess energy range between 2900 and 4500 cm −1 of the S 1 state using quantum beat spectroscopy. A rotational state dependence of S 1-T 1 interaction has been established. The number of interacting triplet states, determined by quantum beat counting, is shown to increase with the rotational quantum number N, and with the average eigenstate lifetime. This effect is attributed to K state mixing in the triplet state which is proposed to be induced by hyperfine interaction via the dipole-dipole term. Furthermore, the nonradiative decay rates exhibit a weak excess energy dependence up to a sudden fluorescence breakdown at ≈4000 cm −1 where they increase dramatically. The break-down, located at 86.6±0.3 kcal/mol, is attributed to the onset of the predissociation due to the radical dissociation HCCCHO→HCCCO+H which proceeds on the S 0 potential energy surface in competition with the concerted reaction HCCCHO→HCCH+CO already accessible at zero S 1, vibrational excess energy.