The first electronic absorption system of acetaldehyde was recorded in the vapour phase at room temperature. The many-banded spectrum proved to be very complex and it was only at the extreme red edge of the absorption that the pattern became simple enough to analyze. The major experimental requirement was a high pressure × path length (500 Torr × 168 m). Spectra were interpreted in terms of the torsional mode ν′15 and ν″15 attached to the ν′14 out-of-plane bending mode. Both modes were highly Franck–Condon active and the [Formula: see text] band at 27240.4 cm−1 was not directly observed. The barrier to rotation of the methyl group was 618.5 cm−1. More surprising was the observation that the methyl group undergoes a rotation from a [Formula: see text] eclipsed configuration to a staggered configuration. The intensity of the ν′14 quantum addition and its position in the spectrum suggest that the aldehydic hydrogen is nonplanar in the ã state.