Fully rotationally resolved spectra of the two lowest-frequency bands ~the origin (00 0 ) and the inversion (140 1) band! of the S1(n p*)ÃS0 transition of jet-cooled (Trot'6 K) acetaldehyde, CH3CHO, have been recorded with a resolution of '0.01 cm21 using a pulsed dye amplified continuous wave ~cw! laser. In modeling the spectra a nonperturbative solution of the rotational– torsional coupling problem was used, but the torsion–inversion and the rotation–inversion couplings were neglected. All the lines of the origin band were reproduced with a model using the same rotational–torsional Hamiltonian for the ground- and excited electronic state. The inversion band could not be described with this model, since the ordering of the torsional levels in the excited state is reversed. The measured spectrum was reproduced by using a rigid asymmetric rotor Hamiltonian for the two torsional levels in the excited state. Some rotational levels of 00 and 141 states were found to be shifted from their predicted energy values. These shifts were explained by an accidental resonance between the excited singlet level and some higher rovibronic triplet level. The relative intensities of the ab-type and c-type torsional subbands and the Herzberg–Teller– induced transition dipole moment direction, characterizing the ab-type subband, were determined by an intensity fit.