The microwave spectrum of heptan-2-one (CH3COC4H8CH3) was recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. The two energetically most favorable conformers could be identified and fitted with standard deviations close to measurement accuracy. The splittings arising from the internal rotations of two methyl groups, the acetyl methyl group CH3CO- and the pentyl methyl group -C4H8CH3, could be resolved. The barriers to internal rotation of the acetyl methyl group are 185.666(14) and 233.418(32) cm(-1) for conformer I and II, respectively, and can be linked to their specific geometries. For the pentyl methyl group, the respective barrier heights are 982.22(86) and 979.2(21) cm.(-1).