Abstract The microwave spectrum of acetaldehyde has been investigated in the frequency range from 7 to 40 GHz. A rather complete assignment of rotational transitions in the ground and torsionally excited states has been found with the help of microwave-microwave double resonance techniques. The spectral data have been analyzed using three different models for the overall and internal rotation problem including a nonrigid model. The threefold component of the internal rotation barrier was determined to be V 3 = 400 ± 2 cm −1. The sixfold contribution V 6 = −10.9 ± 0.3 cm −1 could only be adjusted reliably from data for both ground and torsionally excited states using the nonrigid model. The methods of barrier determinations have been critically reviewed. In an appendix, the Hamiltonian for a nonrigid model is derived based on structure relaxation of the methyl top during internal rotation.