Hybrid free rotor-harmonic oscillator basis functions are used for the variational study of the vibrational coupling between the methyl torsion and the aldehydic hydrogen wagging + CCO bending motions in the S-0 state of acetaldehyde. The kinetic terms and the potential function are expressed as a three-dimensional symmetry-adapted Fourier + Taylor series expansions. The data for the derivation of the kinetic and potential functions were obtained from ab initio calculations at the MP2(full)/6-311G (d,p) level. The use of hybrid basis functions reduces the size of the Hamiltonian matrices. Thus, sizes of 140, 135, and 250 were used for the a(1), a(2), and e representations of the nonrigid group of the molecule. A perturbation is found between the e components of the fourth level of the torsional mode and the first quantum of bending. As a consequence, the a(1)-e unperturbed sequence for the two first torsional energy levels attached to the first quantum of bending is reversed. The separation between these energy levels increases from 0.06 to 2.67 cm(-1).