The rotational spectra of two conformers of pentan-2-one (also known as methyl propyl ketone) were recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. Fine splittings due to the internal rotations of the acetyl methyl group CH3CO- and the butyryl methyl group -COCH2CH2CH3 were resolved and analyzed with high accuracy. The torsional barriers of the acetyl and the butyryl methyl groups are 238.14 and 979 cm(-1), respectively, for the lowest energy conformer, as well as 188.384 and 1032 cm(-1), respectively, for the other one. From the results obtained for the acetyl methyl group, a general rule to predict its torsional barrier in ketones based on the molecular symmetry is proposed.