We studied the state-resolved dynamics of S1 acetaldehyde to product channels with quantum-beat spectroscopy. Two bands near the threshold of dissociation to radical products CH3+HCO in a supersonic jet, displaying most quantum-beat features, are recorded with resolution 0.025 cm−1. Evaluated on the basis of a simple asymmetric rotor, the origins of these two bands 1402−1501 and another denoted # are 31 275.045(1) and 31 523.263(1) cm−1; effective rotational constants of excited state are A=5.7883(1), 5.0408(3), B=0.33269(2), 0.32320(2) and C=0.31026(2), 0.32091(2) cm−1, respectively; large A value results from lack of consideration of torsional motion. For these two vibrational levels most rotational states (about 70 percent) display quantum-beat features attributed to coherently excited singlet–triplet eigenstates. The linewidth in transformed spectra for level #, ∼125 cm−1 below the dissociation threshold, increases with increasing total angular momentum J whereas level 142−151 that is 375 cm−1 below shows a small linewidth independent of J. This is because correlation of the triplet state with dissociation to form radical products results in a decreased lifetime of the triplet state in the tunneling region. A systematic dependence on rotational quantum number implies Coriolis-induced vibrational coupling of triplet states to dissociating continuum on the exit side of the dissociation barrier.