Abstract The two-dimensional torsional potential function for the methyl internal rotors in acetone, CH 3COCH 3, was derived by a global least-squares fit to spectroscopic data only. The data consisted of the torsional transitions in the far infrared and two-photon photoionization spectra and the torsionally split rotational transitions. This new potential function avoids previous contradictions in the values for the effective barrier to internal rotation. It contains potential top–top coupling terms as large as 66% of the effective barrier of 251±3 cm −1. It is similar to the potential function predicted in recent ab initio calculations. The combination of data from rotational and vibrational spectroscopy was crucial to the achievement of a potential function that is compatible with both kinds of data.