AbstractThe rotational spectra of two isotopologues of the 1:1 complex formed between acetone and ethanol have been recorded and analyzed by using Fourier‐transform microwave spectroscopy. One rotamer was detected, in which ethanol adopts the gauche form. The two subunits are linked by a conventional O−H⋅⋅⋅O and a weak C−H⋅⋅⋅O hydrogen bond, forming a six‐membered ring. Each rotational transition is split into five component lines due to the internal rotations of two nonequivalent acetone methyl groups. The V3 barriers to internal rotation of the two CH3 tops of acetone were determined to be 252(4) and 220(1) cm−1, which are noticeably lower than the value for the monomer (266 cm−1).