Author Institution: Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas; Optical Technology Division, National Institute of Standards and TechnologyLabeling problems for rotational energy levels of asymmetric-top molecules containing a noncoaxial internal rotor are investigated by using Rose and Kellman's diabatic correlation methods. We find that vtβ=2 and vtβ=3 rotation-torsion eigenstates for acetaldehyde, which lie just below and just above the barrier to internal rotation, can be unambiguously and meaningfully labeled by nominal quantum numbers Kaβ corresponding to eigenstates of a suitably chosen zeroth-order symmetric-top Hamiltonian. Such a Kaβ labeling is shown to be superior to that based on eigenvector composition for this near-prolate asymmetric rotor. The Kaβ assignments from the diabatic correlation method agree with assignments made using the criterion, based on spectroscopic intuition, of smooth variation of the B value along a series of levels of given Kaβ and increasing J. As a second example, the vtβ=0 state of acetamide lies near the top of the very low barrier in this near oblate top molecule. For these torsion-rotation states also, meaningful Kaβ labels can be determined from the diabatic correlation method. A discussion of the method and of the results obtained for these two example molecules will be presented